Is subnetting only used to less traffic b/c there are fewer broadcast domains? The reason I ask is b/c when you subnet you lose hosts; ie - c class ip no subnet = 254 hosts, c class that gives up 3 bits to subnetting = 240 hosts (16-2=14, 254-14=240). I hope I'm correct in my reasoning.
thx.

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In fact, when you subnet you end up with more, but smaller broadcast domains.

You subnet with /27. You have 8 subnets. You have 30 hosts per subnet.

- IF you use both the subnet-zero and the all-ones subnet, you have 240 hosts;
- IF you use the subnet-zero, but don't use the all-ones subnet, you have 210 hosts;
- if you don't use the subnet-zero and the all-ones subnet, you have 180 hosts.

Yes, it's true you loose some hosts, but consider this. You don't have subnets; you're classful. Some company needs 1000 IP addresses. Chances are the company will request a class B address. Eventually the company will grow to 2000 IP addresses. How many addresses are wasted?!

Cheers!

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A good reason for using subnets is to reduce the size of a broadcast domain. Broadcasts are sent to all hosts on a network or subnetwork. Using routers to divide your network into groups that share servers would be more efficient. You can also use acl's for more security.

--Mark

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If you were to consider a class c address of say 195.100.100.0 with a mask of 255.255.255.224 or /27 if you like, including ip subnet zero, you would have 8 subnets with 30 hosts per subnet. The subnets are:
195.100.100.0
195.100.100.32
195.100.100.64
.
.
.
195.100.100.224

Each of these subnets could be a specific area of your organisation, such as, Sales, Marketing, Engineering etc, and using a Catalyst switch (or two or more) each subnet becomes a separate VLAN. This makes life easier for the poor techie who has to troubleshoot the network, and eases administration, particularly when it comes to expanding the network. Of course with a bigger network, maybe a class b addressing scheme.

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Subnetting is also used for security. I can group all my financial folks on one subnet. In fact in my office this is common place. We have several "secure" networks. This is of course using access-lists.

Reamer

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In addition to the above, since you have smaller broadcast domains with subnetting, you also increase network performance.

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quote:

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Originally posted by Hippo:
[B]If you were to consider a class c address of say 195.100.100.0 with a mask of 255.255.255.224 or /27 if you like, including ip subnet zero, you would have 8 subnets with 30 hosts per subnet. The subnets are:
195.100.100.0
195.100.100.32
195.100.100.64
.
.
.
195.100.100.224
B]

---

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Thanks for all the input. I guess when I posted my orig. question I was mostly thinking about c class subnets and the fact that your hosts are so limited just going with the default mask, subnetting takes even more hosts. Thanks a bunch again.

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