[SATLUG] Netmask /32 - is it usable?

Henry Pugsley henry.pugsley at gmail.com
Sat Jun 2 16:58:01 CDT 2012


On Sat, Jun 2, 2012 at 10:52 AM, Don Wright <satlug at sbcglobal.net> wrote:
> The question came up on a Debian list about supporting the /32 netmask
> (255.255.255.255). I can't see any way to run a TCP session on such a network,
> but since the list has been quiet, I'll throw it out for discussion.
>
> Is there any practical use for an interface that can't talk to anyone else? Or
> is there a pathway I'm not seeing?  --Don

/32 addresses are generally used on loopback interfaces for network
devices.  If a router has multiple interfaces it is easier to have a
single address in order to access the router regardless of what
interfaces are active.  It also provides a single address to use for
sourcing SNMP traps and other management traffic.  By default, routers
will use the interface closest to the destination for sourcing the
traffic, the lowest numbered interface, or some other method which may
result in the source address changing arbitrarily.

The other place they are used is for proxy arp on dialup servers.
Since the IP address could be assigned to a user that may connect to
any number of dialup servers, it would not be feasible to statically
assign IP addresses to a particular device.  When a user connects, the
dialup device advertises the /32 route for the users connection which
propagates through the network and routes traffic back to that
particular dialup server.

As long as the /32 route is advertised through a routing protocol or
configured as a static route, it can be used just like any other IP
address.

-Henry


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