Quick subnetting Q!!!! PLEASE!

Kcolon1

Ok, this is a very easy question that just slipped my mind!!

Class C address with the subnet mask of 255.255.255.0
--How many subnets?
--How many usable host addresses?

Class C address with subnet mask of 255.255.255.254
--How many subnets?
--How many usable host addresses?

I know this is Subnetting 101, but it's just something I'm doubting. Please help!!!

srg

Kcolon1 wrote:

Class C address with the subnet mask of 255.255.255.0
--How many subnets?
--How many usable host addresses?

Ex. 192.168.1.0/255.255.255.0 will be 1 subnet with 254 usable ips.

Kcolon1 wrote:

Class C address with subnet mask of 255.255.255.254
--How many subnets?
--How many usable host addresses?

Ex 192.168.1.0/255.255.255.254 will be a /31 peer to peer link with 2 usable ips and no broadcast. (RFC3021).
I guess you could make 128 /31 nets from one /24 C-net.. but I'm tired, could be wrong.

http://www.faqs.org/rfcs/rfc3021.html
http://www.rfc-archive.org/getrfc.php?rfc=3021

itdaddy

Kcolon1

the first one I confirm
the second with 255.255.255.254 is an invalid mask. type it in google.
I believe it is invalid. you do not have any usable nothing

Robert
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srg

itdaddy wrote:

the second with 255.255.255.254 is an invalid mask. type it in google.
I believe it is invalid. you do not have any usable nothing

Like i said.. you have two usable ips for a point-to-point link. Read up on RFC3021.

Address:   192.168.1.0          11000000.10101000.00000001.0000000 0
Netmask:   255.255.255.254 = 31 11111111.11111111.11111111.1111111 0
Wildcard:  0.0.0.1              00000000.00000000.00000000.0000000 1
=>
Network:   192.168.1.0/31       11000000.10101000.00000001.0000000 0
HostMin:   192.168.1.0          11000000.10101000.00000001.0000000 0
HostMax:   192.168.1.1          11000000.10101000.00000001.0000000 1
Hosts/Net: 2                     Class C, Private Internet, PtP Link RFC 3021

Kcolon1

So I guess /31 is best used for a router-to-router link? Best way to save Addresses, right?

mikej412

Kcolon1 wrote:

So I guess /31 is best used for a router-to-router link? Best way to save Addresses, right?

It works, and Cisco supports it.

But the /31 is one of those things we didn't tell the "old 640-801 CCNAs" about -- not sure if it's used or mentioned in the new 640-802 books or courseware.

So yes, CCNPs do keep secrets from CCNAs.