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Tdcook
@adelphia.net

Tdcook to Tdcook

Anon

to Tdcook

Re: [Other] Vlsm

Thank you for taking the time for such a detailed explination. I am going to ask one more question to just make sure I understood it correctly. If I were to take a network address, say, 195.5.5.0, and subnet it a few different times, am I ever going to be able to get more than 254 usable host addresses out of it simply because they would have different subnet masks, or is this still a max, and VLMS is just a way to use as many of these 254 addresses as you can, and prevent groups of them from going to waste because you don't have that many hosts on all your subnets? Wow... That was a long question. Just incase you don't understand that question, let me ask another one that should help me. Can I EVER have two IP addresses that are the same, EXCEPT for a different mask (Ex. 172.16.64.1/27 and 172.16.64.1/30) [I got these two by not stopping at the end number for each original subnet, but continuing to subnet it all the way to the end, so that I has originally subnetted 172.16.0.0 using a 27 bit mask, and the first number comes from the 1's subnet (0.0 - 31.255) however, I did not stop there, but kept subnetting it. This is a little hard to type because I don't understand why or how this could be done myself. Wow... If the first question had everyone a little confused...

Matt3
All noise, no signal.
Premium Member
join:2003-07-20
Jamestown, NC

Matt3

Premium Member

TDCook:

No and No.

ccie8122
join:2003-11-20
Bountiful, UT

ccie8122 to Tdcook

Member

to Tdcook
Not only is the answer to you question "no" as previously answered, but when you subnet up a network, you actually lose addresses.

The reason is this:

Think of 192.168.1.0/24. You have a total of 256 address on that network, from 192.168.1.0 to 192.168.1.255. As you know, only 254 are usable, because .0 is the network address, and .255 is the broadcast address.

Suppose you subnet the beast in half. Now you have subnetworks 192.168.1.0/25 and 192.168.1.128/25. Well now you have two network numbers: .0 and .128 and two bcast addies: .127 and .255. So now you only have 252 usable addies-- 126 on each subnet-- instead of 254.

Take this to the ultimate extreme, and subnet out to /31 and you have 0 usable addresses. Even numbers would all be network numbers, and odd numbers would all be bcast addies.

This is why when you carve up a subnet, as most orgs do, for point-to-point links (e.g. serial links between two routers), they always use /30s. This divides a Class C up into 64 subnets, each with 4 address: a network addy, bcast addy, and 2 usable -- one for the router at each end of the link.

If you think about it, though, with 64 networks, each with 2 usable and 2 unusable addresses, you now have only half of the address space usable in that network. The other 128 addresses are overhead, unusable.

BTW, great answer "Wr T."

HTH

kr