How do you get loopback addresses other than 127.0.0.1 to work on OS X

Solution:

Here is the short answer: sudo ifconfig lo0 alias 127.0.0.* up

Each alias must be added individually (sudo ifconfig lo0 alias 127.0.0.2 up, sudo ifconfig lo0 alias 127.0.0.3 up). It can be done manually for testing, or a subset or the complete list of the other 250 available numbers in that subnet can be made into StartupItems script that will do it automagically at boot time.

The long answer:
According to RFC3330,
127.0.0.0/8 – This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere.

For those interested, here is a little bash script that adds all the aliases for the IPs 127.0.0.*:

#!/bin/bash
for ((i=2;i<256;i++))
do
    sudo ifconfig lo0 alias 127.0.0.$i up
done