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I am not sure if this is the right place to make this question but I thought this is one of the most appropriate SE's site to make a question on networking. Please move it to other more appropriate sites. Thanks.

So suppose I have a block of IPv4 addresses and one of the hosts address is 182.44.82.16/26. Is it possible for me to find the first and last address of this block of address?

I am thinking like:

182.44.82.16 is 182.44.82.(0001 0000) where the (0001 0000) is the binary for 16.

Since the mask is /26, I thought I could do this:

IP:   182.44.82.(0001 0000)
Mask: 182.44.82.(1111 1000)

And then this means I left with only the last 3 bits for the IP addresses, which makes the first address going to be 182.44.82.16 and the last address 182.44.82.23. But this doesn't look quite right. How can I calculate the first and last addresses of this block of addresses?

xenon
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4 Answers4

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/26 means 26 bits for the subnet. At 8 bits per byte you get 24 for the first three bytes, and two for the last. It means your last byte will be of the form (nnhh hhhh), where n is a subnet bit and h a host bit. 16 is indeed (0001 0000), so you get (00xx xxxx).

Thus the network address is (00|00 0000) -> .0, the broadcast is (00|11 1111) -> .63, and the first and last useable addresses are .1 and .62.

b0fh
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IPv4 addresses are 32-bit unsigned INTs.

Since both the address and netmask can be expressed as a 32-bit unsigned INT, and they are intimately related, this is easy:

first = (addr && netmask)
last = (addr && netmask) + !netmask
adaptr
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  • Is the `!` sign flipping the bits? Eg: !(101101) = 010010, is this right? – xenon Nov 04 '11 at 13:09
  • Correct. I was using C semantics, where ! means NOT. It is equivalent to (MAXINT XOR netmask), which may make it more clear for you. – adaptr Nov 04 '11 at 13:11
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Subnet Calculator might work, http://www.subnet-calculator.com/. Assuming the network class is class 'B' you get 182.44.82.0 - 182.44.82.63

Eggbox06
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  • Why assume ? He stated the CIDR netmask is /26. Really, "classes" are not exactly relevant anymore. – adaptr Nov 04 '11 at 13:20
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as given - any block of IP address is 182.44.82.16/26 where /26 define the Prefix and to find First addr and Last addr you should know these formula:

First addrs = any IP addrs AND Netmask

Last addrs = any IP addrs OR ( ! Netmask )

to find Netmask :- as u given the Prefix is /26 that means 26 1's there Netmask = 11111111 11111111 1111111 11000000 (255.255.255.192)

! Netmask= 00000000 00000000 00000000 00111111

Now First addrs- 182.44.82.16 AND 255.255.255.192 you will get First addrs = 182.44.82.0

and Last addrs- 182.44.82.16 OR ( ! Netmask) you will get Last addrs = 182.44.82.63

(use binary calculation to get these ans)

Deer Hunter
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Sumit Butola
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