Access control lists

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Tags:
Cisco
Networking
Routers
I am a bit confused about writing access control lists, for example how do you write an access list so that only the even number of IP addresses are allowed in and another one to allow the odd number of IP addresses. Also how does one one split an ip subnet address in half, allowing top half full access and the bottom half different access. We can use 212.219.243.70 /27. Any guides or online material will help.
ASKED: February 21, 2005  9:48 AM
UPDATED: February 22, 2005  2:30 PM

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What software are you using? There are several good books for cisco and linux firewalls. Here is a link for iptables: http://www.siliconvalleyccie.com/linux-hn/iptables-intro.htm
For your question on even and odd addresses, I hope you mean two adjacent ranges and not every other actual IP address. The rules don’t work that way.
Splitting up a network is very much like subnetting. With a cisco pix you specify the base address and a dotted decimal netmask to show the range of addresses the rule applies to. With linux you can also use the address/bitmask form. The message I want you to get here is to divide your users into contiguous groups on binary borders. If you can’t do this you will have to add many extra rules to catch the exceptions.
For your example lets look at a typical rule:
iptables -A FORWARD -s 0/0 -i eth0 -d 212.219.243.70/28 -o eth1 -p TCP –sport 1024:65535 –dport 80 -j ACCEPT
This rule allows HTTP traffic, (destination port 80), from anywhere using unprivileged source ports to reach server addresses in the lower half of your range. It also specifies that the traffic allowed is coming in on eth0 and going out on eth1. In most of your rules you probably won’t need this many options but they are there in case you do.
Another issue is the default rules. Cisco has an implicit default deny at the end of the access list. Iptables policies can be set to drop, (recommended), or accept. Keep the policies in mind as you create your lists.

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