Subnetting Class C Addresses
The following table shows you the subnet, valid host, and broadcast address of the first four and last four subnets in the 255.255.255.252 Class C subnet:
Practice Example #6C: 255.255.255.128 (/25)
This mask can be used when you need two subnets, each with 126 hosts. But our trusty big five questions won’t work with this one—it’s special—so I’ll just explain it to you. First, use the global configuration command ip subnet-zero to tell your router to break the rules and allow the use of the first and last subnets, which have subnet bits of all 0s and all 1s (this is a default command on all routers running the 12.x Cisco IOS). The ability to support a 1 subnet-bit mask is an added benefit of this command.
Since 128 is 10000000 in binary, there is only 1 bit for subnetting. Since this bit can be either off or on, the two available subnets are 0 and 128. You can determine the subnet value by looking at the decimal value of the fourth octet. If the value of the fourth octet is below 128, then the host is in the 0 subnet. If the fourth octet value is above 128, then the host is in the 128 subnet.
The following table shows you the two subnets, valid host ranges, and broadcast addresses for the Class C 255.255.255.128 (/25) mask:
So, if you have an IP address of 192.168.10.5 using the 255.255.255.128 subnet mask, you know it’s in the range of the 0 subnet and bit number 128 must be off. If you have an IP address of 192.168.10.189, then 128 must be on, and the host is considered to be in the 128 subnet. You’ll see this again in a minute.
Subnetting in Your Head: Class C Addresses
It really is possible to subnet in your head. Even if you don’t believe me, I’ll show you how. And it’s not all that hard either—take the following example:
= Node address
= Subnet mask
First, determine the subnet and broadcast address of the above IP address. You can do this by answering question 3 of the big five questions: 256 – 224 = 32. 0, 32 + 32 = 64. The address of 33 falls between the two subnets of 32 and 64 and must be part of the 192.168.10.32 subnet.