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watchmansky
Asked: 2016-10-14 09:18:30 +0800 CST

127.0.0.1/8 and CIDR notation

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After a deep review of network and CIDR notation I often see in linux config file:

127.0.0.1/8

to set network loopback interface.

The correct CIDR should be 

127.0.0.0/8

Because 127.0.0.1 is the loopback IP. Why many default linux and bsd config file use the first?

linux

3 Answers

  1. Looking at the first example, 127.0.0.1/8, it's quite clear that it has bits set in the part of the address that is not part of the prefix (first 8 bits, as specified by /8).
    If the goal is to just define the prefix that identifies this network, it doesn't really make any sense to include those bits, and 127.0.0.0/8 (or even 127/8) would be the way to go instead.

    However, if the goal is to specify a host address and the network's prefix length all in one go (a somewhat more modern take on the classic host IP address + netmask), 127.0.0.1/8 makes perfect sense.

    It's not clear from the question but I would like to think that you have seen this in contexts where a host address is being specified.
    The other alternative would seem to be that someone has made a mistake.

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  2. In CIDR notation 127.0.0.1/8 is a means of specifying a host address and subnet mask. 127.0.0.0/8 is a means of specifying a network address and subnet mask. They're two different things, both expressed in CIDR notation.

    If I say that my network address is 192.168.1.0/24 and my host address is 192.168.1.1/24 then that means that my network address is 192.168.1.0 with a subnet mask of 255.255.255.0 and my host address is 192.168.1.1 with a subnet mask of 255.255.255.0. CIDR notation can be used to represent both, as your example implies.

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  3. Every address in the 127.0.0.0/8 address range is an IPv4 loopback address, not just 127.0.0.1.

    In many contexts, it is completely proper to refer to an address as 127.0.0.1/8 instead of the old way of using the address and mask, e.g. 127.0.0.1 255.0.0.0.

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