subnet mask of /32

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I have seen on some ISPs an IP address and a subnet mask of
255.255.255.255 (/32)

I'm a little confused of this as one needs a default gateway. The
default gateway of this network would be???

This to me is a network address and broadcast address, so I'm not sure
how to understand this.

Does anyone have a proper explanation of this?

Regards

James

Re: subnet mask of /32

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Hi James

This type of IP address is often known as a loopback address, normally
issued to a loopback interface on your router for management purpose. The
router announces the particular IP address via its routing protocol to
other routers. Actually some routing protocols demand such an IP address
on a loopback for proper functionallity.

A loopback interface is actually a virtual interface on the router, that
never goes down unless you explicit do a shutdown on the interface.

Regards,
Lars Christensen
CCIE #20292

Re: subnet mask of /32
James wrote:
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The default route would be the interface itself.

ip route 0.0.0.0 0.0.0.0 Dialer0

or

ip route 0.0.0.0 0.0.0.0 BVI1

Re: subnet mask of /32
I use /32 loopbacks but have never thought about it in depth as to how
and why.

So this would count for an internet provider giving you a /32 as well?
Thr interface is the default route out?

Joe Beasley wrote:
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Re: subnet mask of /32
A subnet mask of 255.255.255.255 or /32 represents a single address.
This can commonly be found in routing tables as an entry for a route to a
single IP address.

EXAMPLE:
A router within the internal routing protocol (IGP) domain, such as EIGRP or
OSPF, might have two FastEthernet interfaces.  One FastEthernet interface
could lead to an Internet connection and have an IP address of
192.168.42.254 and a subnet mask of 255.255.255.0 (/24).  The other
FastEthernet interface could lead to another Internet connection have an IP
address of 192.168.59.254 and a subnet mask of 255.255.255.0 (/24).
In this situation, the routing table of a neighbor router to this Internet
border router would receive a route for 192.168.42.0/255.255.255.0 and
192.168.59.0/255.255.255.0.
Now the Internet border router configures a static default route through the
first Internet connection and a static route for traffic to a single
destination IP address through the second Internet connection.  The static
route for the default route would be for 0.0.0.0/0.0.0.0 and the static
route for the individual IP address could be something like
172.16.15.14/255.255.255.255.
After this static route configuration and redistribution into the IGP, the
neigbor router now sees a route for 172.16.15.14/32 in its routing table.
The effect is that all traffic not matching a route table entry will match
the default route and go to the Internet border router to use the first
Internet interface.  Traffic specifically to 172.16.15.14 will go to the
Internet border router and use the second Internet interface.

-----
Scott Perry
Indianapolis, IN
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