Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

According to everything I read, the entire 127.x.x.x subnet should loopback.

However, on my Mac, I can only ping 127.0.0.1

I know I've done this before (though possibly on another OS) and has come in very useful for developing multiple SSL sites locally and for tunneling remote services for access on a local IP (for example I could ssh into my MySQL server, and just port forward the standard port to the same port on my local machine but on 127.0.0.2 while my local server ran at 127.0.0.1.

share|improve this question
up vote 33 down vote accepted
+50

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.

share|improve this answer
3  
If only there was a way to make it work for 127.*.*.* – Matthew Schinckel Dec 12 '12 at 4:27
    
If this turns out to also work on CentOS, you sir are getting a bounty from me. – Parthian Shot Jul 18 '14 at 19:33
    
Actually, either way you get a bounty. Since it doesn't work for CentOS, I'll just ask and answer that question once I find out the answer. – Parthian Shot Jul 18 '14 at 19:42

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
share|improve this answer
    
Am I reading this wrong? What you created looks like it does all the addresses from 127.0.0.1 to 127.0.0.255. If you change "sudo ifconfig lo0 alias 127.0.0.$i up" to "sudo ifconfig lo0 alias 127.$i.$i.$i up" that'll get you most of them right? However you would skip anything that is less than 2.. So maybe use a different variable for the other two octets? The highest IP address for loopback is 127.255.255.255 – Everett Dec 18 '15 at 14:45
    
@Everett, you read this correctly, it only does the addresses 127.0.0.*. The reason I didn't cover the whole range is that it is actually quite slow to process even 255 addresses, so 255x255x255 would take way too long. – this.lau_ Dec 18 '15 at 14:47
1  
Thanks. I just wanted to make sure that the expectation was set, because someone will come on here and say, "Hey, it didn't work for 127.7.53.91." I wouldn't do that, but someone will... – Everett Dec 18 '15 at 14:49
    
Actually, @Everett's proposal wouldn't work either, since it would only enable adresses such as 127.7.7.7 or 127.42.42.42, that is all those with the three last digits are equals. You would need three distinct loops to actually enable all IPs in the 127/8 block. Yet doing this would certainly not be a good idea: it is highly probable, given that each IPs has to be enabled separately, that there are some resources attached to each assigned IP; allocating approximately 17.7 millions IPs (that is 256^3) might turn out to have some significant consequences... – jwatkins May 16 at 16:49

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .