255.255.255.255 - as subnetmask

Im sorry, I must not understand. When you say you can use it on any interface you perfer? If you put it on a Windows XP or Windows Vista box it will throw a IP conflict error, and shut down tcp/ip stack. If I put it on a Cisco router (12.4) it immediately loses layer 3. I would agree its a valid logical subnet, but in what instance would it be a valid subnet for a interface. I guess im asking when would you use it for anything other then a VIF or Logical interface?

...I would agree its a valid logical subnet, but in what instance would it be a valid subnet for a interface.

As I said earlier, I've used it when configuring a loopback address i.e. 172.16.1.1 255.255.255.255

[code:1]Router(config)#int fa0/0
Router(config-if)#ip add 192.168.2.1 255.255.255.255
Bad mask /32 for address 192.168.2.1
Router(config-if)#no shut[/code:1]

And for a loop back.

[code:1]Router(config)#int loopback0
Router(config-if)#ip add 192.168.1.1 255.255.255.255
Router(config-if)#no shut[/code:1]

The resultant config:

[code:1]interface Loopback0
ip address 192.168.1.1 255.255.255.255
!
interface FastEthernet0/0
no ip address
duplex auto
speed auto
![/code:1]

Windows XP refused to accept both /32 and /31 masks.

I dont mean to sound argumentative. Correct me if I am wrong, but I believe you are talking about configuring a loopback on a cisco router, which are Logical Interfaces. I mealy ment to make the point that if you use that subnet you can have nothing else in the subnet. The original question implied that it was for PPoE, which would require 2 address available in the subnet.

Cisco documentation does not list /32 as a valid subnet (Ref: www.cisco.com/en/US/tech/tk365/technolog...186a0080093f33.shtml )

I mealy ment to make the point that if you use that subnet you can have nothing else in the subnet. The original question implied that it was for PPoE, which would require 2 address available in the subnet.

I totally agree with you Kajitora

his original question was concerning PPP - if you are using PPPo[A/E] at home (e.g. DSL) check your connection and you will see you've been assigned a /32 host route. Its normal for PPP and its kinda cool because you can have routers on different subnets talk to each other over a Point to Point link.

Why are using PPPoE? Seems like a real hassle for a LAN. And yes by the way, if a host wants to talk to another it will do so thru its default gateway. The edge router (default gateway) the host connects to will have routes for the network and will advertise the /32's to the network as a aggregate route.