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Loopback's subnet mask

Can someone explain the use of the loopback subnet mask.

Cisco's documentation reports many different options and is not consistent.

I.E

Interface loopback 20

ip address 192.168.20.1 255.255.255.0

---OR---

Interface loopback 20

ip address 192.168.20.1 255.255.255.252

---OR---

Interface loopback 20

ip address 192.168.20.1 255.255.255.255

If the loopback interface is only used for management, (and by many different protocols for many different reasons but Not routing), should this always be /32? Is there a method to the madness? Is there a reason to have something other than a /32 on the loopback interface?

Thanks again!

Frank

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Participant

Re: Loopback's subnet mask

If you have tons of address space to waste then it's okay to give them a subnet.

There is no hard and fast rule to use a 32 bit mask or any other mask for that matter, it all depends on your requirments.

Say for example you wanted to use a loopback and had an interface in the box, you could assign a 30 bit mask to the loopback and use "ip unnumbered loopback x on the interface's". This was you don't really have to waste address space adn could make a interface appear as a decent subnet.

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