There is more than one localhost

November 20, 2023

Well, technically I lied. Of course, localhost maps to just 127.0.0.1 but that is not the only address that points to your local machine. There are actually 16.7 million (2^24) addresses, as the loopback network range is 127.0.0.0/8.

I find it particularly useful to have different loopback addresses per project, while keeping their ports consistent. For example:

Project A development environment

127.0.0.2:3000 http server
127.0.0.2:5432 postgres
127.0.0.2:6379 redis

Project B development environment

127.0.0.3:3000 http server
127.0.0.3:5432 postgres
127.0.0.3:6379 redis

It’s also worth mentioning that the zeroes can be omitted, like this

127.0.1.1 --> 127.1.1
127.0.0.1 --> 127.1

That’s normally a thing you see with IPv6 addresses, where the loopback range is ::1/128 (just one address).

0000:0000:0000:0000:0000:0000:0000:0001 --> ::1

Update: There is a recent proposal (2024-12-29) that aims to narrow 127.0.0.0/8 down to 127.0.0.0/16. Even if that goes through, you still have 65536 addresses (2^16) to play with, which is plenty.

Tags:

Networks