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Why is localhost IP 127.0.0.1?

Why is localhost IP 127.0.0.1?

I wondered what is the origin of the decision to make localhost's IP address 127.0.0.1. What is the "meaning" of 127? what is the "meaning" of 0.0.1?

Asked by: Guest | Views: 34
Total answers/comments: 3
Guest [Entry]

127 is the last network number in a class A network with a subnet mask of 255.0.0.0. 127.0.0.1 is the first assignable address in the subnet. 127.0.0.0 cannot be used because that would be the wire number. But using any other numbers for the host portion should work fine and revert to using 127.0.0.1. You can try it yourself by pinging 127.1.1.1 if you'd like. Why they waited until the last network number to implement this? I don't think it's documented.
Guest [Entry]

"The designers of the Internet really knew how the hardware worked, and they designed with low level implementation in mind.

The values 0, 127 and 255 are special in 8 bit assembly and machine language programming because there are ""tricks"" you can use to test for these values and branch to different code using smaller instructions that execute faster than for other integers. 127 is the highest signed 8 bit integer, so incrementing it by 1 will cause a signed overflow. Similarly, incrementing 255 will cause unsigned overflow. Merely loading the value 0 into a register will usually set a zero flag on the chip. Imagine the networking program looks like this in pseudocode:

if (value == 0) doLocal();
if (value == 127) doLoopback();
if (value == 255) doNetwork();

Although it depends on the chip, in those days most chips could encode these tests with 2 words, 3 words and 3 words respectively (total 8 words) and further those particular tests were all likely to execute in 1 clock cycle each. Using any other value would probably require 4 words each (total 12 words), a 50% increase in code size and likely a 50% increase in execution time as well."
Guest [Entry]

"First, the whole 127.x.x.x range points to your localhost.
127 in binary is ""01111111"". ""11111111"" = 255 and 0 are reserved, so the choice is obvious :)"