Subnet mask of /31?

[JEDI]

/31 = 2 addresses

but each subnet has a broadcast addr, and a subnet addr. thus a /31 usually don't have any usable addr. (i guess if you set the box to classless, you can use the subnet addr and have 1 usable addr.)

Is a /31 practical in the real world?

 

[spidey07]

you got it. There are no addresses with a /31 - only the network and broadcast.

Hence it is never used.

Although /30s are used all the time for WAN links where only two IPs are needed.

 

[bgroff]

Sure there are /31s. Its the latest and greatest in WAN addressing. The subnet mask would be 255.255.255.254. Since the modern routing protocols use multicast instead of broadcast, it works. If you are using Cisco IOS 12.2T or better, it will accept a /31 network address.

 

[bgroff]

Double posting... Such a faux pas.

 

[spidey07]

Originally posted by: bgroff
Sure there are /31s. Its the latest and greatest in WAN addressing. The subnet mask would be 255.255.255.254. Since the modern routing protocols use multicast instead of broadcast, it works. If you are using Cisco IOS 12.2T or better, it will accept a /31 network address.

huh?

 

[bgroff]

Originally posted by: spidey07

Originally posted by: bgroff
Sure there are /31s. Its the latest and greatest in WAN addressing. The subnet mask would be 255.255.255.254. Since the modern routing protocols use multicast instead of broadcast, it works. If you are using Cisco IOS 12.2T or better, it will accept a /31 network address.

huh?

I'm not making it up.
Check it.

 

[spidey07]

Originally posted by: bgroff

Originally posted by: spidey07

Originally posted by: bgroff
Sure there are /31s. Its the latest and greatest in WAN addressing. The subnet mask would be 255.255.255.254. Since the modern routing protocols use multicast instead of broadcast, it works. If you are using Cisco IOS 12.2T or better, it will accept a /31 network address.

huh?

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080087aeb.html

what about inverse arp?

OH, nevermind. Its not a standard...still in proposed stage. gotcha

 

[bgroff]

Apparently its old info... The RFC looks to have been published. Check it.

 

[spidey07]

Originally posted by: bgroff
Apparently its old info... The RFC looks to have been published. Check it.

meh
I still don't like it. Would probably reak havoc on management systems.

:evil:

 

[bgroff]

Originally posted by: spidey07

Originally posted by: bgroff
Apparently its old info... The RFC looks to have been published. Check it.

meh
I still don't like it. Would probably reak havoc on management systems.

:evil:

Nah, it works like a champ. I've seen it deployed on a global MPLS network for the serial interlinks. Its quite disconcerting to see it in the config for the first time tho... Just looks weird to see:

interface serial2/0:0
ip address 192.168.1.0 255.255.255.254

The upside is you get to double your address space in the wan links for free.

 

[cmetz]

A /31 is perfectly fine on non-broadcast networks.

For BSD-like networking stacks, this literally means !IFF_BROADCAST, which means don't treat the all-ones and all-zeros host parts specially.

Given that directed broadcast is considered harmful and more and more devices don't do it anyway, we should stop wasting the addresses.

 

[narzy]

I'd still use a /30 your not loosing anything and your effectivly doubling your availible IP channels (*I SOO made that last part up.)