Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.
The Question
SuperUser reader Disa is curious about loopback IPs:
What kind of information can we tease out from this table?
What’s the difference between them? Can they be used interchangeably?
The Answer
Two SuperUser contributors jumped in to help solve the mystery. First, Mmmc offers this succinct overview:
Then, YLearn offers a broader overview of naming conventions in general and how to think about them:
127.0.0.1 is a loopback address 127.0.0.2 is a loopback address 127.0.0.3 is a loopback address and so on
127.0.0.0 is a network address. Together with mask 255.0.0.0 it gives you a hint that whole class A of addresses starting with 127...* will contain loopback addresses.
In some ways this is similar to how we navigate in real life.
The first column provides the list of known destinations (where can I go) and the second column indicates how specific the destination (I can go to Canada or I can go to Uncle John’s house in Canada). Without getting into great detail, the “higher” the mask value, the more specific the destination. So a value of 0.0.0.0 covers going to any device and a value of 255.255.255.255 specifies an individual device.
The third column specifies where traffic should go next to get to the destination (if you are going to Canada, you need to start by getting on Main Street) and the fourth column indicates which path out of the device should be used to get to the destination (from home you may only have your driveway but from the Walmart parking lot you may have several “exits” to choose from).
Finally, the metric gives the computer a way to choose the best path if there are multiple routes to the destination (you can go out either the north or east exit from the parking lot to get to Canada, but the east one is a faster).
So to answer the original question, no you can’t use 127.0.0.0 and 127.0.0.1 interchangeably. The difference shown here is that there exists two routes – a general route to any device using 127.x.y.z and a very specific route to host 127.0.0.1 (which is in 127.0.0.0), both of which use the 127.0.0.1 interface
