Subnetting in Your Head: Class C Addresses
Written on 15:46 by Ahmet Emir
It absolutely is accessible to subnet in your head. Even if you don’t accept me, I’ll appearance you how.
And it’s not all that adamantine either—take the afterward example:
192.168.10.33 = Bulge address
255.255.255.224 = Subnet mask
Subnet 0 4 8 12 … 240 244 248 252
First host 1 5 9 13 … 241 245 249 253
Last host 2 6 10 14 … 242 246 250 254
Broadcast 3 7 11 15 … 243 247 251 255
Should We Absolutely Use This Affectation That Provides Alone Two Hosts?
You are the arrangement ambassador for Acme Corporation in San Francisco, with dozens of
WAN links abutting to your accumulated office. Appropriate now your arrangement is a classful network,
which agency that the aforementioned subnet affectation is on anniversary host and router interface. You’ve read
about classless acquisition breadth you can accept altered admeasurement masks but don’t apperceive what to use
on your point-to-point WAN links. Is the 255.255.255.252 (/30) a accessible affectation in this situation?
Yes, this is a actual accessible affectation in advanced breadth networks.
If you use the 255.255.255.0 mask, again anniversary arrangement would accept 254 hosts, but you alone use 2
addresses with a WAN link! That is a decay of 252 hosts per subnet. If you use the 255.255.255.252
mask, again anniversary subnet has alone 2 hosts and you don’t decay adored addresses. This is a really
important subject, one that we’ll abode in a lot added detail in the area on VLSM network
design after in this chapter.
Subnetting Basics 125
First, actuate the subnet and advertisement abode of the aloft IP address. You can do this
by answering catechism 3 of the big bristles questions: 256 – 224 = 32. 0, 32, 64. The abode of
33 avalanche amid the two subnets of 32 and 64 and charge be allotment of the 192.168.10.32 subnet.
The abutting subnet is 64, so the advertisement abode of the 32 subnet is 63. (Remember that the
broadcast abode of a subnet is consistently the cardinal appropriate afore the abutting subnet.) The valid
host ambit is 33–62 (the numbers amid the subnet and advertisement address). This is too easy!
Okay, let’s try addition one. We’ll subnet addition Class C address:
192.168.10.33 = Bulge address
255.255.255.240 = Subnet mask
What subnet and advertisement abode is the aloft IP abode a affiliate of? 256 – 240 = 16.
0, 16, 32, 48. Bingo—the host abode is amid the 32 and 48 subnets. The subnet is
192.168.10.32, and the advertisement abode is 47 (the abutting subnet is 48). The accurate host range
is 33–46 (the numbers amid the subnet cardinal and the advertisement address).
Okay, we charge to do more, aloof to accomplish abiding you accept this down.
You accept a bulge abode of 192.168.10.174 with a affectation of 255.255.255.240. What is the
valid host range?
The affectation is 240, so we’d do a 256 – 240 = 16. This is our block size. Aloof accumulate abacus 16
until we canyon the host abode of 174, starting at zero, of course: 0, 16, 32, 48, 64, 80, 96, 112,
128, 144, 160, 176. The host abode of 174 is amid 160 and 176, so the subnet is 160. The
broadcast abode is 175; the accurate host ambit is 161–174. That was a boxy one.
One more—just for fun. This is the easiest one of all Class C subnetting:
192.168.10.17 = Bulge address
255.255.255.252 = Subnet mask
What subnet and advertisement abode is the aloft IP abode a affiliate of? 256 – 252 = 0
(always alpha at aught unless told otherwise), 4, 8, 12, 16, 20, etc. You’ve got it! The host address
is amid the 16 and 20 subnets. The subnet is 192.168.10.16, and the advertisement abode is 19.
The accurate host ambit is 17–18.
Now that you’re all over Class C subnetting, let’s move on to Class B subnetting. But before
we do, let’s accept a quick review.
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