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Chris R
Asked: 2010-09-29 14:01:07 +0800 CST

What does this route map modification mean?

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I've inherited administration duties for a firewall/gateway server, and I'm trying to understand what a particular static route setup in rc.local means:

...
route add 123.123.123.123/30 10.10.3.14
route add 123.123.123.123/32 10.10.3.13
...

Obviously, the IP addresses have been changed to protect the innocent.

What does this mean, and why is it routed this way?

networking static-routes route

3 Answers

  1. Best Answer

    It means packets for 123.123.123.120-122 will go via 10.10.3.14 and packets for 123.123.123.123 will go via 10.10.3.13.

    • Since the advent of CIDR, netmasks indicate the length of the prefix - technically any IP in the network can be used, but by convention the non-masked bits are set to 0, which is why 'network addresses' are always on even numbers.

    • Most specific wins - /32 is more specific (longer) than /30, so it takes precedence over the /30.

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  2. Are the "123.123.123.123" addresses really the exact same in the setup you are debugging - or are they slightly different?

    Assuming they are different - the first line means that anything in the network defined by 123.123.123.123/30 should be forwarded to 10.10.3.14. The second line means the specific IP address 123.123.123.123 should be forwarded to 10.10.3.13.

    As to why - we'd need more context.

    • Can you tell me the last octet (the last .123) specifically - what exactly is it in your situation. Having two of htem the same with two different netmasks is a bit weird, and could mean a couple of different things....
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  3. First line: packets to hosts at 123.123.123.123/30 are routed via 10.10.3.14, except (second line) for packets to 123.123.123.123/32, which are routed via 10.10.3.13.

    However, I suspect a anonymization error, if you really define a /30 network, the first address (same ip/32) is the "network"-address, which doesn't really get packets. It could also be, that someone tries to route a slice out of a bigger subnet differently, but it smells funny...

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