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Home > Archive > CCNA > September 2003 > Subnetting 255.255.255.0 & 255.255.255.128
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Subnetting 255.255.255.0 & 255.255.255.128
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| digitalsoul 2003-09-14, 11:30 pm |
| 1. Contrary to public belief, 255.255.255.0 is not a class C subnet mask. This is a class B subnet mask with 8 bits of subnetting (Todd Lammle CCNA Guide, 2nd edition)
2. And How do we subnet 255.255.255.128?
Can someone please clarify how we subnet these two? I understand subnetting well, but these two I can't grasp well. | |
| martek 2003-09-15, 11:50 am |
| 1. 255.255.255.0 is the default subnet mask for a C class network, where the first octet is between 192 and 223.
It is also a B class subnet mask where 8 bits are used for subnets, or an A class subnet mask where 16 bits are used for subnets.
2. A 255.255.255.128 subnet mask will give you 2 subnets with a C class network (though in theory both are reserved and thus invalid). The same mask will give you 512 subnets (minus 2 reserved) with a B class network, and 131072 subnets (minus 2 reserved) with an A class network.
Though all this classful stuff is somewhat outdated. I suggest you look into things like CIDR (Classless Interdomain Routing) and VLSM (Variable Length Subnet Masks) for a broader view.  | |
| joel316 2003-09-15, 4:38 pm |
| 1. Don't know where you got the information that 255.255.255.0 is a Class B subnet mask because it isn't the only time you will see this is if you get an IP like 172.16.1.0 255.255.255.0 which means it's using 8 bit for the subnet mask. This means that there will only be 1 Subnet with 256-2=254 Hosts.
2. 255.255.255.128 Class C.
If you look at it this way, 255.255.255.0 should be the default. Now your taken 1 bit of the host for subnetting which means it could be written /25 which means that there 7 bits left for host 2^7=128-2=126 Host which means your Subnet address would be something like 192.168.1.128 and then you have hosts from 129 to 254 and then you have your Broadcast address would be 192.168.1.255.
I believe this is right. Correct me if I'm wrong anyone. |
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