Practice Archetype #6B: 255.255.255.128 (/25)
Written on 23:22 by Ahmet Emir
This is one of the hardest subnet masks you can comedy with. And worse, it absolutely is a really
good subnet to use in assembly because it creates over 500 subnets with 126 hosts for each
subnet—a nice mixture. So, don’t skip over it!
172.16.0.0 = Network address
255.255.255.128 = Subnet mask
Subnet 0.0 2.0 4.0 6.0 8.0
First host 0.1 2.1 4.1 6.1 8.1
Last host 1.254 3.254 5.254 7.254 9.254
Broadcast 1.255 3.255 5.255 7.255 9.255
Subnet 0.0 1.0 2.0 3.0 ... 254.0 255.0
First host 0.1 1.1 2.1 3.1 ... 254.1 255.1
Last host 0.254 1.254 2.254 3.254 ... 254.254 255.254
Broadcast 0.255 1.255 2.255 3.255 ... 254.255 255.255
Subnetting Basics 131
Subnets? 29 = 512.
Hosts? 27 – 2 = 126.
Accurate subnets? Okay, now for the catchy part. 256 – 255 = 1. 0, 1, 2, 3, etc. for the third
octet. But you can’t balloon the one subnet bit acclimated in the fourth octet. Remember back I
showed you how to amount one subnet bit with a Class C mask? You amount this the same
way. (Now you apperceive why I showed you the 1-bit subnet affectation in the Class C section—
to accomplish this allotment easier.) You absolutely get two subnets for anniversary third octet value, hence
the 512 subnets. For example, if the third octet is assuming subnet 3, the two subnets
would absolutely be 3.0 and 3.128.
Broadcast abode for anniversary subnet?
Accurate hosts?
The afterward table shows how you can actualize subnets, accurate hosts, and broadcast
addresses application the Class B 255.255.255.128 subnet affectation (the aboriginal eight subnets are shown,
and again the aftermost two subnets):
Subnet 0.0 0.128 1.0 1.128 2.0 2.128 3.0 3.128 ... 255.0 255.128
First
host
0.1 0.129 1.1 1.129 2.1 2.129 3.1 3.129 ... 255.1 255.129
Last
host
0.126 0.254 1.126 1.254 2.126 2.254 3.126 3.254 ... 255.126 255.254
Broadcast
0.127 0.255 1.127 1.255 2.127 2.255 3.127 3.255 ... 255.127 255.255
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