Re: internet ip addressing
- From: ibuprofin@xxxxxxxxxxxxxxxxxxxxxx (Moe Trin)
- Date: Tue, 05 Sep 2006 15:05:34 -0500
On Tue, 05 Sep 2006, in the Usenet newsgroup comp.os.linux.networking, in
article <edj65j$q51$1@xxxxxxxxxxxxxxxxxx>, Giovanni wrote:
Jeroen Geilman wrote:
jasonsig wrote:
Hi, with regards to /31 addressing on point to point links. I have not
seen any real world examples using this type of addressing scheme
That would be because it is impossible.
The minimum number of adresses needed for a valid subnet is 3.
(1 network, 1 host, 1 gateway)
You need also a broadcast address making the minimum number to four.
In the conventional - non point-to-point link, four addresses are the
minimum. Required is the broadcast. There may be two OR THREE hosts
(some operating systems do not need the "network address" and can use
it as a host address).
For point-to-point networks, the broadcast is not needed, which is why
in most implementations, they are /32s. For example, pppd will _show_
a network mask
[conover ~]$ /sbin/ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr:198.18.180.190 P-t-P:192.168.195.11 Mask:255.0.0.0
UP POINTOPOINT RUNNING MTU:1500 Metric:1
but compare that to the output for your ethernet interface[s]
[conover ~]$ /sbin/ifconfig eth0
eth0 Link encap:10Mbps Ethernet HWaddr 00:44:17:56:ea:17
inet addr:192.0.2.117 Bcast:192.0.2.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
notice the "POINTOPOINT" compared to "BROADCAST"
What is normal in a point to point link is that the "client" end will
have a unique address - 198.18.180.190 in the example above, while the
"server" (could be a terminal server with a hundred serial ports) _MAY_
use a single address for all links - 192.168.195.11 the example above.
Were you to look at the routing table on such a server, there would be
no confusion of which "port" is used to reach any of the clients.
RFC3021 is a proposed solution where the two ends of a point to point
link have unique (but consecutive) IP addresses. I'm not aware of anyone
actually doing this, but the RFC exists. They are using a /31 (two IP
addresses) because this is a point to point link, and neither a broadcast
or network address is needed for the link to work.
Old guy
.
- References:
- Re: internet ip addressing
- From: Jeroen Geilman
- Re: internet ip addressing
- From: Giovanni
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