subnet mask of 255.255.255.0

[manishrathi]

If subnet mask is 255.255.255.0, does it mean that for a network with this subnet mask can have only one subnet with 256 hosts ?

Likewise, if subnet mask is 255.255.0.0, then does it mean that for a network with this subnet mask can have only one subnet, with 65536 hosts ?

thanks

[bchur83]

Yes, that is correct.

[SpaceRangerJoe]

With a class C (255.255.255.0) you will actually get 254 usable host addresses, not the full 256. The 255 at the end is the subnet broadcast address, so you don't get to use that one. The first IP (.0) is something else you don't get. I'm sure someone hear can tell you, since I don't remember!

[thehacker010]

Actually, this is not true. Subnet Zero can be used. In fact, very large networks should use Subnet Zero to help conserve IP address resources. A lot of higher end Cisco routers have "ip subnet-zero" enabled in the console.

However, for situations that don't require getting every IP address possible, Subnet Zero is not used. The main reason being that people don't know it can actually be used and it can cause confusion if they don't know that and someone tries to.

*Edit*
I completely forgot to tell you guys why Subnet Zero used to not be used. Before Classless Inter-Domain Routers were used, subnets were broken into parts. "192.168.12.[0-63], 192.168.12.[64-127],
192.168.12.[128-191], and 192.168.12.[192-255]" . Because CIDR did not exist, you could not use the first or last "part" of the subnet, which means only "192.168.12.[64-127] and 192.168.12.[128-191]" could actually be used. I believe this is a trick question on the old Cisco cert exams. I think it was changed now to reflect the new use of subnet zero. Don't quote me on that though.

[Drew@PSU]

thehacker010 is quite correct, in all accounts. They did indeed change the Cisco cert exams to reflect the use of "ip subnet-zero".

As a little bit more info, the last IP in a subnet ( .255 ) is the subnet broadcast, as SpaceRangerJoe mentioned, and the first ( .0 ) can be referred to as the subnet number, depending on the "ip subnet-zero" setting.