>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".
Kind of a nit, but 1122 has been superceded by 1812.
>
>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.
>
>
>________________________________________________
>Get your own "800" number
>Voicemail, fax, email, and a lot more
>http://www.ureach.com/reg/tag
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=17403&t=17343
--------------------------------------------------
FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
