24 June, 2011

non-cidr netmask: "worked in testing but broke colleagues' brains"

CIDR prefix length (for example, the 24 in is a more concise notation for (a commonly used subset of) netmasks.

A prefix length contains less information - it can only represent netmasks that consist of a sequence of 1 bits, followed by 0 bits to the end. For example, /24 is 11111111111111111111111100000000 (24 1s and then 32-24=8 0s)

This is useful because thats how most people use netmasks.

But there's a set of netmasks that aren't representable this way - for example 11111111000000001111111100000000.

Did anyone ever use netmasks that weren't prefix-length-representable? Apparently yes:

Addresses were allocated from these networks sequentially, and the oldest
web sites tended to get the most traffic, so a straightforward setup that
spread the six /18s across the reverse proxies didn't balance the load
particularly well. I toyed with using 0xffff0003 netmasks to split the /16
so that successive addresses could be routed to each of the four London
reverse proxies in turn.

This worked in testing but I didn't deploy it because it broke my
colleagues' brains and non-contiguous netmasks were an unsupported

No comments:

Post a Comment