ZyXEL Communications P-663HN-51 Manual

P-663HN-51 Support Notes
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All contents copyright © 2009 ZyXEL Communications Corporation.
P-663HN-51
802.11n ADSL2+ Bonded 4-Port Gateway
Support Notes
Version 3.70
Jul. 2009

P-663HN-51 Support Notes
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All contents copyright © 2009 ZyXEL Communications Corporation.
FAQ................................................................................... 5
General FAQ.............................................................................6
1.How do I access the P-663HN-51 Command Line Interface (CLI)?.......6
2.How do I update the firmware and configuration file?............................6
3. How do I upgrade/backup the firmware by using TFTP client program
via LAN?...................................................................................................6
4. How do I restore P-663HN-51 configurations by using TFTP client
program via LAN?.....................................................................................6
5. What should I do if I forget the system password? ...............................7
6. How to use the Reset button?...............................................................7
7. What is NAT virtual server? When should I use NAT virtual server?....7
8. What is NAT Port Triggering?...............................................................8
9. When should I use NAT Port Triggering? .............................................8
10. How many network users can the NAT support?................................8
Product FAQ.............................................................................8
1. How can I manage P-663HN-51?.........................................................8
2. What is the default password for Web Configurator?............................9
3. What’s the difference between ‘Common User Account’ and
‘Administrator Account’? ...........................................................................9
4. How do I know the P-663HN-51's WAN IP address assigned by the ISP?
...................................................................................................9
5. What is the micro filter or splitter used for?...........................................9
6. The P-663HN-51 supports five types of network protocol for IP over
Ethernet as WAN interface, what's the difference between MER(MAC
Encapsulation Routing) and Bridge? ......................................................10
7. How do I know I am using PPPoE? ....................................................10
8. Why does my provider use PPPoE?...................................................10
9. What is DDNS?...................................................................................10
10. When do I need DDNS service?.......................................................11
11. What is DDNS wildcard? Does the P-663HN-51 support DDNS
wildcard?.................................................................................................11
12. What is Traffic Shaping?...................................................................11
13. Why do we perform traffic shaping in the P-663HN-51?...................12
14. What do the parameters (PCR, SCR, MBS) mean? .........................12
15. What do the ATM QoS Types (CBR, UBR, VBR-nRT, VBR-RT) mean?
.................................................................................................12
16. What is IP filtering?...........................................................................13
17. Can the P-663HN-51's handle IPSec packets sent by the IPSec
gateway? ................................................................................................13

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18. How do I setup my P-663HN-51 for routing IPSec packets over NAT?
.................................................................................................13
ADSL FAQ...............................................................................14
1. How does ADSL compare to Cable modems?....................................14
2. What is the expected throughput? ......................................................15
3. What is the microfilter used for? .........................................................15
4. How do I know the ADSL line is up?...................................................15
5. How does the P-663HN-51 work on a noisy ADSL?...........................15
6. Does the VC-based multiplexing perform better than the LLC-based
multiplexing?...........................................................................................15
7. How do I know the details of my ADSL line statistics?........................16
8. What are the signaling pins of the ADSL connector?..........................16
9. How do I use single DSL line?............................................................17
Firewall FAQ...........................................................................18
General....................................................................................18
1. What is a network firewall?.................................................................18
2. What makes P-663HN-51 secure?.....................................................18
3. What are the basic types of firewalls? ................................................18
4. Why do you need a firewall when your router has packet filtering and
NAT built-in?...........................................................................................19
5. What is Denials of Service (DoS) attack?...........................................19
6. What is Ping of Death attack?.............................................................19
7. What is Teardrop attack?....................................................................20
8. What is SYN Flood attack?.................................................................20
9. What is LAND attack?.........................................................................20
10. What is Brute-force attack?...............................................................20
11. What is IP Spoofing attack?..............................................................21
Configuration..........................................................................22
1. How do I configure the firewall?..........................................................22
2. How do I control some PC’s time of accessing to Internet?................22
3. Why can't I configure my P-663HN-51 using Web Configurator/Telnet
over WAN? .............................................................................................23
4. Why can't I upload the firmware and configuration file using FTP over
WAN? .................................................................................................24
Log and Alert.......................................................................................25
1. When does the P-663HN-51 generate the firewall log?......................25
2. What does the log show to us?...........................................................26
3. When does the P-663HN-51 generate the firewall alert?....................26
4. What is the difference between the log and alert?..............................26
Wireless FAQ..........................................................................27

P-663HN-51 Support Notes
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General FAQ.......................................................................................27
1. What is a Wireless LAN?....................................................................27
2. What are the advantages of Wireless LAN?.......................................27
3. What is the disadvantage of Wireless LAN?.......................................28
4. Where can you find 802.11 wireless networks?..................................28
5. What is an Access Point?...................................................................28
6. Is it possible to use wireless products from a variety of vendors? ......28
7. What is Wi-Fi? ....................................................................................28
8. What types of devices use the 2.4GHz Band?....................................28
9. Does the 802.11 interfere with Bluetooth device?...............................29
10. Can radio signals pass through wall? ...............................................29
11. What are potential factors that may causes interference among WLAN
products?................................................................................................29
12. What's the difference between a WLAN and a WWAN?...................30
13. Can I manually swap the wireless module without damage any
hardware?...............................................................................................30
14. What wireless security mode does P-663HN-51 support?................30
15. What Wireless standard does P-663HN-51 support?.......................30
16. Does P-663HN-51 support MAC filtering?........................................30
17. Does P-663HN-51 support auto rate adaption?................................30
Advanced FAQ .......................................................................31
1. What is Ad Hoc mode?.......................................................................31
2. What is Infrastructure mode?..............................................................31
3. How many Access Points are required in a given area?.....................31
4. What is Direct-Sequence Spread Spectrum Technology –(DSSS)?..31
5. What is Frequency-hopping Spread Spectrum Technology –(FHSS)?..
.................................................................................................31
6. Do I need the same kind of antenna on both sides of a link? .............32
7. Why the 2.4 GHZ Frequency range?..................................................32
8. What is Server Set ID (SSID)?............................................................32
9. What is an ESSID?.............................................................................32
Security FAQ ..........................................................................33
1. How do I secure the data across the P-663HN-51 Access Point's radio
link? .................................................................................................33
2. What is WEP?.....................................................................................33
3. What is WPA-PSK?............................................................................33
4. What is the difference between 40-bit and 64-bit WEP?.....................33
5. What is a WEP key?...........................................................................34
6. Will 128-bit WEP communicate with 64-bit WEP?..............................34
7. Can the SSID be encrypted?..............................................................34

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8. By turning off the broadcast of SSID, can someone still sniff the SSID?
.................................................................................................34
9. What are Insertion Attacks?................................................................34
10. What is Wireless Sniffer?..................................................................34
Application Notes...................................................................36
General Application Notes...................................................................36
1. Internet Access Using P-663HN-51 under Bridge mode.....................36
2. Internet Access Using P-663HN-51 under Routing mode...................41
3. Setup the P-663HN-51 as a DHCP Relay ..........................................45
4. Using the Dynamic DNS (DDNS)........................................................46
5. Using syslog .......................................................................................48
6. Using IP Alias .....................................................................................49
7. Using Static Route..............................................................................50
8. RIP .................................................................................................51
9. Using IGMP Multicast .........................................................................53
10. About Certificates .............................................................................53
11. Interface Group.................................................................................54
12. Change WAN MTU via WEB-GUI.....................................................54
Wireless Application Notes..................................................................56
1. Configure a Wireless Client to Ad hoc mode ......................................56
2. MAC Filter...........................................................................................61
3. IEEE 802.11n......................................................................................62
4. Site Survey .........................................................................................63
5. Configure 802.1x and WPA ................................................................66
6. The WPS/WLAN Button......................................................................71
7. What’s the difference between IEEE802.11a/b/g/n?...........................72
Support Tool...........................................................................73
1. LAN/WAN Packet Trace .....................................................................73
2. Firmware/Configurations Uploading and Downloading using TFTP....78
3. Using FTP to Upload the Firmware and Configuration Files...............81
Command Syntax and General User Interface.....................84
FAQ

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General FAQ
1. How do I access the P-663HN-51 Command Line Interface (CLI)?
The Command Line Interface is for the Administrator use only, and it could be
accessed via telnet session.
Note: It is protected by super password, ‘1234’by factory default.
2. How do I update the firmware and configuration file?
You can do this if you access the P-663HN-51 as Administrator. You can
upload the firmware and configuration file to Prestige from Web Condigurator,
or using FTP or TFTP client software. You CAN NOT upload the firmware and
configuration file via Telnet because the Telnet connection will be dropped
during uploading the firmware. Please do not power off the router right after the
FTP or TFTP uploading is finished, the router will upload the firmware to its
flash at this moment.
Note: There may be firmware that could not be upgraded from Web
Configurator. In this case, ZyXEL will prepare special Upload Software
for you. Please read the firmware release note carefully when you want to
upload a new fireware.
3. How do I upgrade/backup the firmware by using TFTP client program
via LAN?
The P-663HN-51 allows you to transfer the firmware to P-663HN-51 using
TFTP program via LAN. The procedure for uploading firmware via TFTP is as
follows.
a. Use the TELNET client program in your PC to login to your
P-663HN-51.
b. Enter CI command 'sys stdio 0' to disable Stdio idle timeout
c. To upgrade firmware, use TFTP client program to put firmware in file
'ras' in the Prestige. After data transfer is finished, the P-663HN-51 will
program the upgraded firmware into FLASH ROM and reboot itself.
d. To backup your firmware, use the TFTP client program to get file 'ras'
from the Prestige.
4. How do I restore P-663HN-51 configurations by using TFTP client
program via LAN?
a. Use the TELNET client program in your PC to login to your
P-663HN-51.

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b. Enter CI command 'sys stdio 0' disable Stdio idle timeout
c. To backup the P-663HN-51 configurations, use TFTP client program to
get file 'rom-0' from the P-663HN-51.
d. To restore the P-663HN-51 configurations, use the TFTP client program
to put your configuration in file rom-0 in the P-663HN-51.
5. What should I do if I forget the system password?
In case you forget the system password, you can erase the current
configuration and restore factory defaults this way:
Use the RESET button on the rear panel of P-663HN-51 to reset the router.
After the router is reset, the LAN IP address will be reset to '192.168.1.1', the
common user password will be reset to 'user', the Administrator password will
be reset to ‘1234’.
6. How to use the Reset button?
a. Turn your P-663HN-51 on. Make sure the POWER led is on (not
blinking)
b. Press the RESET button for longer than one second and shorter than
ten seconds and release it.
c. Press the RESET button for ten seconds and then release it. If the
POWER LED begins to blink, the default configuration has been
restored and the P-663HN-51 restarts.
7. What is NAT virtual server? When should I use NAT virtual server?
NAT virtual server (port forwarding) entries are to have the P-663HN-51
forward traffic from the WAN to LAN computers.
You might do this to get particular games or services to work through NAT.
You can also make servers, for example, web or FTP, visible to the outside
world even though NAT makes your whole inside network appear as a single
computer to the outside world.
You may enter a single port number or a range of port numbers to be
forwarded, and the local IP address of the desired server. The port number
identifies a service; for example, web service is on port 80 and FTP on port 21.
In some cases, such as for unknown services or where one server can support
more than one service (for example: both FTP and web service), it might be

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better to specify a range of port numbers. You can allocate a server IP address
that corresponds to a port or a range of ports.
8. What is NAT Port Triggering?
Trigger port forwarding allows computers on the LAN to dynamically take turns
using port forwarding service. P-663HN-51 records the IP address of a LAN
computer that sends traffic to the WAN to request a service with a specific port
number and protocol (a "trigger" port).
When P-663HN-51's WAN port receives a response with a specific port
number and protocol ("incoming" port), it forwards the traffic to the LAN IP
address of the computer that sent the request. After that computer’s
connection for that service closes, another computer on the LAN can use the
service in the same manner. This way you do not need to configure a new IP
address each time you want a different LAN computer to use the application.
9. When should I use NAT Port Triggering?
Some services use a dedicated range of ports on the client side and a
dedicated range of ports on the server side. With regular port forwarding you
set a forwarding port in NAT to forward a service (coming in from the server on
the WAN) to the IP address of a computer on the client side (LAN).
The problem is that port forwarding only forwards a service to a single LAN IP
address. In order to use the same service on a different LAN computer, you
have to manually replace the LAN computer's IP address in the forwarding port
with another LAN computer's IP address.
10. How many network users can the NAT support?
P-663HN-51 do limit the number of the sessions. It can support 3960 sessions
that you can use.
Product FAQ
1. How can I manage P-663HN-51?
Multilingual Embedded Web GUI for Local and Remote management

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CLI (Command-line interface)
Telnet support (Administrator Password Protected ) for remote
configuration change and status monitoring
FTP/ TFTP sever, firmware upgrade and configuration backup and
restore are supported(Administrator Password Protected)
2. What is the default password for Web Configurator?
There are two different accounts for P-663HN-51 Web Configurator: Common
User Account and Administrator Account.
By factory default the password for the two accounts are:
Common User Account: user
Administrator Account: 1234.
You can change the password after you logging in the Web Configurator.
Please record your new password whenever you change it. The system
will lock you out if you have forgotten your password.
3. What’s the difference between ‘Common User Account’and
‘Administrator Account’?
For Common User Account, it can only access the status monitor of
P-663HN-51 and check the current system status.
For Administrator Account, besides accessing the status monitor of
P-663HN-51, it can also access Wizard setup/ Advanced setup of
P-663HN-51:
Moreover, only with Administrator Password, you could manage the
P-663HN-51 via FTP/TFTP or Telnet.
4. How do I know the P-663HN-51's WAN IP address assigned by the ISP?
You can view Device Info>>WAN, IP address is shown in WAN Info>>IPv4
Address.
5. What is the micro filter or splitter used for?
Generally, the voice band uses the lower frequency ranging from 0 to 4KHz,
while ADSL data transmission uses the higher frequency. The micro filter acts
as a low-pass filter for your telephone set to ensure that ADSL transmissions
do not interfere with your voice transmissions. For the details about how to
connect the micro filter please refer to the user's manual.

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6. The P-663HN-51 supports five types of network protocol for IP over
Ethernet as WAN interface, what's the difference between MER(MAC
Encapsulation Routing) and Bridge?
When the ISP limits some specific computers to access Internet, that means
only the traffic to/from these computers will be forwarded and the other will be
filtered. In this case, we use bridge mode which works as an ADSL modem to
connect to the ISP. The ISP will generally give one Internet account and limit
only one computer to access the Internet.
For most Internet users having multiple computers want to share an Internet
account for Internet access, they have to add another Internet sharing device,
like a router. In this case, we use the router mode which works as a general
Router plus an ADSL Modem.
7. How do I know I am using PPPoE?
PPPoE requires a user account to login to the provider's server. If you need to
configure a user name and password on your computer to connect to the ISP
you are probably using PPPoE. If you are simply connected to the Internet
when you turn on your computer, you probably are not. You can also check
your ISP or the information sheet given by the ISP. Please choose PPPoE as
the encapsulation type in the P-663HN-51 if the ISP uses PPPoE.
8. Why does my provider use PPPoE?
PPPoE emulates a familiar Dial-Up connection. It allows your ISP to provide
services using their existing network configuration over the broadband
connections. Besides, PPPoE supports a broad range of existing applications
and service including authentication, accounting, secure access and
configuration management.
9. What is DDNS?
The Dynamic DNS service allows you to alias a dynamic IP address to a static
hostname, allowing your computer to be more easily accessed from various
locations on the Internet. To use the service, you must first apply an account
from several free Web servers such as http://www.dyndns.org/.
Without DDNS, we always tell the users to use the WAN IP of the P-663HN-51
to reach our internal server. It is inconvenient for the users if this IP is dynamic.
With DDNS supported by the P-663HN-51, you apply a DNS name (e.g.,

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www.zyxel.com.tw) for your server (e.g., Web server) from a DDNS server.
The outside users can always access the web server using the
www.zyxel.com.tw regardless of the WAN IP of the P-663HN-51.
When the ISP assigns the P-663HN-51 a new IP, the P-663HN-51 updates
this IP to DDNS server so that the server can update its IP-to-DNS entry. Once
the IP-to-DNS table in the DDNS server is updated, the DNS name for your
web server (i.e., www.zyxel.com.tw) is still usable.
10. When do I need DDNS service?
When you want your internal server to be accessed by using DNS name rather
than using the dynamic IP address we can use the DDNS service. The DDNS
server allows to alias a dynamic IP address to a static hostname. Whenever
the ISP assigns you a new IP, the P-663HN-51 sends this IP to the DDNS
server for its updates.
11. What is DDNS wildcard? Does the P-663HN-51 support DDNS
wildcard?
Some DDNS servers support the wildcard feature which allows the hostname,
*.yourhost.dyndns.org, to be aliased to the same IP address as
yourhost.dyndns.org. This feature is useful when there are multiple servers
inside and you want users to be able to use things such as
www.yourhost.dyndns.org and still reach your hostname.
Yes, the P-663HN-51 supports DDNS wildcard that http://www.dyndns.org/
supports. When using wildcard, you simply enter yourhost.dyndns.org in
Advanced Setup > DNS > Dynamic DNS.
12. What is Traffic Shaping?
Traffic Shaping allocates the bandwidth to WAN dynamically and aims at
boosting the efficiency of the bandwidth. If there are serveral VCs in the
P-663HN-51 but only one VC activated at one time, the P-663HN-51 allocates
all the Bandwidth to the VC and the VC gets full bandwidth. If another VCs are
activated later, the bandwidth is yield to other VCs after ward.
Note: Traffic shaping controls outgoing (upstream) traffic, not incoming
(downstream).

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13. Why do we perform traffic shaping in the P-663HN-51?
The P-663HN-51 must manage traffic fairly and provide bandwidth allocation
for different sorts of applications, such as voice, video, and data. All
applications have their own natural bit rate. Large data transactions have a
fluctuating natural bit rate. The P-663HN-51 is able to support variable traffic
among different virtual connections. Certain traffic may be discarded if the
virtual connection experiences congestion. Traffic shaping defines a set of
actions taken by the P-663HN-51 to avoid congestion; traffic shaping takes
measures to adapt to unpredictable fluctuations in traffic flows and other
problems among virtual connections.
14. What do the parameters (PCR, SCR, MBS) mean?
Peak Cell Rate(PCR): The maximum bandwidth allocated to this connection.
The VC connection throughput is limited by PCR.
Sustainable Cell Rate(SCR): The least guaranteed bandwidth of a VC.
When there are multi-VCs on the same line, the VC throughput is guaranteed
by SCR.
Maximum Burst Size(MBS): The amount of cells transmitted through this
VC at the Peak Cell Rate before yielding to other VCs. Total bandwidth of the
line is dedicated to single VC if there is only one VC on the line. However, as
the other VC asking the bandwidth, the MBS defines the maximum number of
cells transmitted via this VC with Peak Cell rate before yielding to other VCs.
The P-663HN-51 holds the parameters for shaping the traffic among its virtual
channels. If you do not need traffic shaping, please set SCR = 0, MBS = 0 and
PCR as the maximum value according to the line rate (for example, 2.3 Mbps
line rate will result PCR as 5424 cell/sec.)
15. What do the ATM QoS Types (CBR, UBR, VBR-nRT, VBR-RT) mean?
These are the basic ATM traffic classes defined by the ATM Forum Traffic
Management 4.0 Specification.
Constant bit rate(CBR): An ATM bandwidth-allocation service that requires
the user to determine a fixed bandwidth requirement at the time the connection
is set up so that the data can be sent in a steady stream. CBR service is often
used when transmitting fixed-rate uncompressed video.

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Unspecified bit rate(UBR): An ATM bandwidth-allocation service that does
not guarantee any throughput levels and uses only available bandwidth. UBR
is often used when transmitting data that can tolerate delays, such as e-mail.
Variable bit rate(VBR): An ATM bandwidth-allocation service that allows
users to specify a throughput capacity (i.e., a peak rate) and a sustained rate
but data is not sent evenly. You can select VBR for bursty traffic and
bandwidth sharing with other applications. It contains two subclasses:
Variable bit rate nonreal time (VBR-nRT):
Variable bit rate real time (VBR-RT):
16. What is IP filtering?
By default, the ZyXEL Device allows all IP outgoing traffic from the LAN to
WAN, and block all IP incoming traffic from WAN to LAN. But you also can use
IP Filtering to setup some rules to make some modification.
17. Can the P-663HN-51's handle IPSec packets sent by the IPSec
gateway?
Yes, the P-663HN-51's NAT can handle IPSec ESP Tunneling mode. We
know when packets go through NAT, NAT will change the source IP address
and source port for the host. To pass IPSec packets, SUA must understand
the ESP packet with protocol number 50, replace the source IP address of the
IPSec gateway to the router's WAN IP address. However, NAT should not
change the source port of the UDP packets which are used for key
managements. Because the remote gateway checks this source port during
connections, the port thus is not allowed to be changed.
18. How do I setup my P-663HN-51 for routing IPSec packets over NAT?
For outgoing IPSec tunnels, no extra setting is required.
For forwarding the inbound IPSec ESP tunnel, A server IP Address is required.
You could configure it in Web Configurator, Advanced Setup -> NAT ->
Virtual Servers:

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It is because NAT makes your LAN appear as a single machine to the outside
world. LAN users are invisible to outside users. So, to make an internal server
for outside access, we must specify the service port and the LAN IP of this
server in Web configurator. Thus NAT is able to forward the incoming packets
to the requested service behind NAT and the outside users access the server
using the P-663HN-51's WAN IP address. So, we have to configure the
internal IPsec client as a default server (unspecified service port) when it acts
a server gateway.
ADSL FAQ
1. How does ADSL compare to Cable modems?
ADSL provides a dedicated service over a single telephone line; cable
modems offer a dedicated service over a shared media. While cable modems

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have greater downstream bandwidth capabilities (up to 24 Mbps), that
bandwidth is shared among all users on a line, and will therefore vary, perhaps
dramatically, as more users in a neighborhood get online at the same time.
Cable modem upstream traffic will in many cases be slower than ADSL, either
because the particular cable modem is inherently slower, or because of rate
reductions caused by contention for upstream bandwidth slots. The big
difference between ADSL and cable modems, however, is the number of lines
available to each. There are no more than 12 million homes passed today that
can support two-way cable modem transmissions, and while the figure also
grows steadily, it will not catch up with telephone lines for many years.
Additionally, many of the older cable networks are not capable of offering a
return channel; consequently, such networks will need significant upgrading
before they can offer high bandwidth services.
2. What is the expected throughput?
In our test, we can get up to 24Mbps(Single Line) or 44Mbps(Dual Lines) in an
ideal environment.
3. What is the microfilter used for?
Generally, the voice band uses the lower frequency ranging from 0 to 4KHz,
while ADSL data transmission uses the higher frequency. The micro filter acts
as a low-pass filter for your telephone set to ensure that ADSL transmissions
do not interfere with your voice transmissions. For the details about how to
connect the micro filter please refer to the user's manual.
4. How do I know the ADSL line is up?
You can see the DSL LEDs Green on the P-663HN-513's front panel is on
when the ADSL physical layer is up.
5. How does the P-663HN-51 work on a noisy ADSL?
Depending on the line quality, the P-663HN-51 uses "Fall Back" and "Fall
Forward" to automatically adjust the date rate.
6. Does the VC-based multiplexing perform better than the LLC-based
multiplexing?
Though the LLC-based multiplexing can carry multiple protocols over a single
VC, it requires extra header information to identify the protocol being carried

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on the virtual circuit (VC). The VC-based multiplexing needs a separate VC for
carrying each protocol but it does not need the extra headers. Therefore, the
VC-based multiplexing is more efficient.
7. How do I know the details of my ADSL line statistics?
You can also do it in Web Configurator:
Device Info -> Statistics -> ADSL Line:
8. What are the signaling pins of the ADSL connector?
The signaling pins on the P-663HN-51's ADSL connector are pin 2, pin 3, pin 4,
and pin 5. DSL 1 is pin 3 and pin 4. DSL 2 is pin 2 and pin 5. The middle 4 pins
for a RJ11 cable.

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9. How do I use single DSL line?
If you want to use single line mode, you need to enable it in Web Configurator:
Advanced Setup -> DSL:

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Firewall FAQ
General
1. What is a network firewall?
A firewall is a system or group of systems that enforces an access-control
policy between two networks. It may also be defined as a mechanism used to
protect a trusted network from an untrusted network. The firewall can be
thought of two mechanisms: One to block the traffic, and the other to permit
traffic.
2. What makes P-663HN-51 secure?
DoS Prevention
Stateful Packet Inspection
ACL between outgoing and incoming.
At least 20 ACL rules
Protocol and Generic Packet Filter
SIP ALG
3. What are the basic types of firewalls?
Conceptually, there are three types of firewalls:
1. Packet Filtering Firewall
2. Application-level Firewall
3. Stateful Inspection Firewall
Packet Filtering Firewalls generally make their decisions based on the header
information in individual packets. These headers information include the
source, destination addresses and ports of the packets.
Application-level Firewalls generally are hosts running proxy servers, which
permit no traffic directly between networks, and which perform logging and
auditing of traffic passing through them. A proxy server is an application
gateway or circuit-level gateway that runs on top of general operating system
such as UNIX or Windows NT. It hides valuable data by requiring users to

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communicate with secure systems by mean of a proxy. A key drawback of this
device is performance.
Stateful Inspection Firewalls restrict access by screening data packets against
defined access rules. They make access control decisions based on IP
address and protocol. They also 'inspect' the session data to assure the
integrity of the connection and to adapt to dynamic protocols. The flexible
nature of Stateful Inspection firewalls generally provides the best speed and
transparency, however, they may lack the granular application level access
control or caching that some proxies support.
4. Why do you need a firewall when your router has packet filtering
and NAT built-in?
With the spectacular growth of the Internet and online access, companies that
do business on the Internet face greater security threats. Although packet filter
and NAT restrict access to particular computers and networks, however, for
the other companies this security may be insufficient, because packets filters
typically cannot maintain session state. Thus, for greater security, a firewall is
considered.
5. What is Denials of Service (DoS) attack?
Denial of Service (DoS) attacks are aimed at devices and networks with a
connection to the Internet. Their goal is not to steal information, but to disable
a device or network so users no longer have access to network resources.
There are four types of DoS attacks:
1. Those that exploits bugs in a TCP/IP implementation such as Ping of
Death and Teardrop.
2. Those that exploits weaknesses in the TCP/IP specification such as
SYN Flood and LAND Attacks.
3. Brute-force attacks that flood a network with useless data such as
Smurf attack.
4. IP Spoofing
6. What is Ping of Death attack?
Ping of Death uses a 'PING' utility to create an IP packet that exceeds the
maximum 65535 bytes of data allowed by the IP specification. The oversize

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packet is then sent to an unsuspecting system. Systems may crash, hang, or
reboot.
7. What is Teardrop attack?
Teardrop attack exploits weakness in the reassemble of the IP packet
fragments. As data is transmitted through a network, IP packets are often
broken up into smaller chunks. Each fragment looks like the original packet
except that it contains an offset field. The Teardrop program creates a series of
IP fragments with overlapping offset fields. When these fragments are
reassembled at the destination, some systems will crash, hang, or reboot.
8. What is SYN Flood attack?
SYN attack floods a targeted system with a series of SYN packets. Each
packet causes the targeted system to issue a SYN-ACK response, While the
targeted system waits for the ACK that follows the SYN-ACK, it queues up all
outstanding SYN-ACK responses on what is known as a backlog queue.
SYN-ACKs are moved off the queue only when an ACK comes back or when
an internal timer (which is set a relatively long intervals) terminates the TCP
three-way handshake. Once the queue is full, the system will ignore all
incoming SYN requests, making the system unavailable for legitimate users.
9. What is LAND attack?
In a LAN attack, hackers flood SYN packets to the network with a spoofed
source IP address of the targeted system. This makes it appear as if the host
computer sent the packets to itself, making the system unavailable while the
target system tries to respond to itself.
10. What is Brute-force attack?
A Brute-force attack, such as 'Smurf' attack, targets a feature in the IP
specification known as directed or subnet broadcasting, to quickly flood the
target network with useless data. A Smurf hacker flood a destination IP
address of each packet is the broadcast address of the network, the router will
broadcast the ICMP echo request packet to all hosts on the network. If there
are numerous hosts, this will create a large amount of ICMP echo request
packet, the resulting ICMP traffic will not only clog up the 'intermediary'
network, but will also congest the network of the spoofed source IP address,
known as the 'victim' network. This flood of broadcast traffic consumes all
available bandwidth, making communications impossible.