This subject is not as clear as it ought to be. If you look at
the subject of loopbacks generically, there are two RFCs that
come to mind. The first deals with RFC 1122 "Requirements for
Internet Hosts". The second deals with RFC 1122 "Requirements
for IPv4 Routers".
For starters, both RFCs do define the address as:
RFC1122 INTERNET LAYER
October 1989
(g) { 127, :any: }
Internal host loopback address. Addresses of
this form MUST NOT appear outside a host.
Baker Standards Track
[Page 47]
RFC 1812 Requirements for IP Version 4 Routers
June 1995
(e) { 127, :any: }
Internal host loopback address. Addresses of this
form MUST NOT appear outside a host.
Now the issue becomes how have Internet hosts adopted this
practice. When you approach it from a host perspective, the
answer is as always, "it depends". In the case of Windows
hosts, they have taken a traditonal view of using 127.0.0.1.
You can see that first by examing the routing table(I hope this
comes out okay):
E:\>route print
================================================================
===========
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 c0 f0 12 ae 56 ...... Novell 2000 Adapter.
================================================================
===========
================================================================
===========
Active Routes:
Network Destination Netmask Gateway
Interface Metric
0.0.0.0 0.0.0.0 192.168.1.1
192.168.1.69 1
127.0.0.0 255.0.0.0 127.0.0.1
127.0.0.1 1
192.168.1.0 255.255.255.0 192.168.1.69
192.168.1.69 1
192.168.1.69 255.255.255.255 127.0.0.1
127.0.0.1 1
192.168.1.255 255.255.255.255 192.168.1.69
192.168.1.69 1
224.0.0.0 224.0.0.0 192.168.1.69
192.168.1.69 1
255.255.255.255 255.255.255.255 192.168.1.69
192.168.1.69 1
================================================================
===========
You will note that any packet with an address destination of
127.x.x.x will be sent to the loopback interface address (which
is defined as 127.0.0.1). Interestingly enough, when you ping
a non-127.0.0.1 IP address on the 127.0.0.0 network, it will
return the same address:
E:\>ping 127.23.45.61
Pinging 127.23.45.61 with 32 bytes of data:
Reply from 127.23.45.61: bytes=32 time 10ms TTL=128
Reply from 127.23.45.61: bytes=32 time 10ms TTL=128
Reply from 127.23.45.61: bytes=32 time 10ms TTL=128
Reply from 127.23.45.61: bytes=32 time 10ms TTL=128
You may be asking why did vendors seem to settle on 127.0.0.1
as the magical address? I don't know. My best answer to that
would be to use a Tim Brown expression and say, "original sin".
More important is why the entire 127.0.0.0 network is used for
loopback and testing. That is a clear and obvious waste of
address space. This gets fixed in IPv6, whereby a single host
address is used for a loopback.
Another point about host loopbacks is key here. This traffic
does not go out on the wire, rather it is kept internal to the
host. You cannot even capture it with sniffer software. The
best way to see the logic diagramm for this and how it works is
to go to the definitive reference on TCP/IP, namely TCP/IP
Illustrated, by W. Richard Stevens. You may want to take a
peek at Figure 2.4 on page 28. If you don't have a copy of
this book, get one.
Next is the issue of Internet gateways (routers). Here, the
lines get a little bit muddied. The reason for this is that
Cisco seems to have adopted a software loopback interface that
goes beyond the definition of a generic loopback interface on
an Internet host. Software loopbacks on the router are not
limited to one (as they typically might be on an Internet
host). The limits on a router are typically those found for
IDBs (interface descriptor blocks). See my previous post in
the archives for the links on the limits of IDBs per platform.
The loopback interfaces on the routers tend to take an
exclusionary view of IP addressing. What I mean by this is
that unless a particular IP address or address range is
prohibited, it can be assigned. For example, see output below:
werner-gateway(config)#int lo 100
werner-gateway(config-if)#ip add 0.2.2.2 255.0.0.0
Not a valid host address - 0.2.2.2
werner-gateway(config-if)#ip add 127.2.2.2 255.0.0.0
Not a valid host address - 127.2.2.2
werner-gateway(config-if)#ip add 127.0.0.1 255.0.0.0
Not a valid host address - 127.0.0.1
werner-gateway(config-if)#ip add 223.0.0.1 255.0.0.0
werner-gateway(config-if)#ip add 224.0.0.1 255.0.0.0
Not a valid host address - 224.0.0.1
werner-gateway(config-if)#ip add 255.0.0.1 255.0.0.0
Not a valid host address - 255.0.0.1
Now the final point you may be wondering is whether 127.0.0.1
is recognized as a valid address on the router for a ping
return to internally check the IP stack on itself. As is usual,
the answer is, "it depends". For example, here is the output
from one of my 2500 routers:
Router#ping 127.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 127.0.0.1, timeout is 2
seconds:
.....
Success rate is 0 percent (0/5)
Now look at the output from my 1601:
werner-gateway#ping 127.0.0.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 127.0.0.2, timeout is 2
seconds:
....
Success rate is 0 percent (0/4)
werner-gateway#ping 127.0.0.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 127.0.0.1, timeout is 2
seconds:
!!!!!
werner-gateway>sh ip int brie
Interface IP-Address OK? Method Status Protocol
Ethernet0 192.168.1.1 YES NVRAM up up
Loopback100 223.0.0.1 YES manual up up
Serial0 209.244.214.43 YES IPCP up up
I hope this clears things up a little bit better.
v/r,
Paul Werner
> >there was a question regarding 127.0.0.1. I understand that
the actual
> >TCP/IP "software" actually uses this address for self
testing.
> >when ping localhost {or computer {netbios} name} the actual
ip address
> of
> >127.0.0.1 shows up on the screen.
> >
> >In doing some research one book explains that 127.0.0.1 is
not useable
> but
> >the rest of the addresses in the 127 network can be used.
> >I went to my trusty 2500 and try to plug in 127.12.12.25 and
the router
> >would not allow me config the interface with that address.
> >
> >Is entire 127 network off limits or just 127.0.0.1. is this
a cisco
> thing.
> >any explaination is appreicated..
>
>
> See RFC 1812. It's only defined to be the one address, but
most
> implementations block the 127 network.
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