# Networking Question - What is a VALID Subnet Address? [duplicate]

Possible Duplicate:
How does IPv4 Subnetting Work?

I am currently revising Networking for the Cisco CCNA1. One of the questions that I am struggling to answer is the following:

• 172.16.4.127 /26
• 172.16.4.155 /26
• 172.16.4.193 /26
• 172.16.4.95 /27
• 172.16.4.159 /27
• 127.16.4.207 /27

I can't get my head around how to know understand when a subnet address is valid or not.

Many thanks

## migrated from stackoverflow.comJun 1 '11 at 19:26

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IP addresses are 32-bits long, the mask is also 32 bits. When you perform a logical `AND` of the address and the mask, you get the subnet address. See this wikipedia section on how to determine the network prefix.

My mental shortcut that works for netmasks >= 25 is I subtract the mask length from 32 and look at the last octet of the IP address... that is the maximum host bits in the address (call that number `h`). If the last octet is evenly divisible by `2**h`, then that is a subnet address.

For example, 172.16.4.127/26... `32 - 26 = 6`. `2**6 = 64` and `127 % 64 = 63`. Therefore, 172.16.4.127 is not a valid subnet address... in fact it is the broadcast address for the 172.16.4.64/26 subnet. Good luck with your CCNA exam.

The answer is either all, or none depending on what they mean.

The normal understanding of a 'valid' subnet address is one in which the address quoted is the lowest possible in the specified range. Hence a /26 (64 addresses) would end with a multiple of 64, and a /27 would end with a multiple of 32.

None of the addresses you've quoted meet that rule.

• 172.16.4.127 /26 - this is the broadcast address for 172.16.4.64 /26
• 172.16.4.155 /26 - this sits in the range 172.16.4.128 - 191
• 172.16.4.193 /26 - this is the first usable address in 172.16.4.192 /26
• 172.16.4.95 /27 - this is the broadcast address for 172.16.4.64 /27
• 172.16.4.159 /27 - this is the broadcast address for 172.16.4.128 /27
• 127.16.4.207 /27 - this sits in the range 172.16.4.192 - 223

Are you sure you copied them correctly?

• I'm not quite sure I'm parsing your answer... you say it could be all or none depending on what they mean... 172.16.4.127/26 is not a valid subnet address... That is the broadcast address, by definition. He is studying for a test, and they throw these kind of problems at them to ensure they understand the concepts correctly. – Mike Pennington Apr 19 '11 at 9:57
• what I'm saying is that none of the supplied values match the usual definition of a "subnet address". – Alnitak Apr 19 '11 at 10:13
• @ Mike Pennington - "to ensure they understand" I find CCNA very difficult, I can pass 90% of the exams, and I can subnet successfull, but I dont understand theses sorts of questions. So for instance 172.16.4.127/26 is the last address before the next subnet meaning that it is a broadcast address, this means that it is not a useable address by a host therefore not a subnet address.Is this correct? – Phil Apr 19 '11 at 16:43

When I get lost with network addresses (I agree, it is not easy to calculate those /26 or /27s), I just ask the ipcalc tool to do the math for me. But be careful, because `ipcalc` in CentOS/RHEL is a completely different tool.

You run `ipcalc` and pass it some kind of network address as an argument and it gives you all kinds of useful self-explanatory information (looking at the binary netmask you can understand what a valid network address is in such a way that you will remember it). Taking your first address as an example (the space before the slash is optional).

``````\$ ipcalc 172.16.4.127 /26
Netmask:   255.255.255.192 = 26 11111111.11111111.11111111.11 000000
Wildcard:  0.0.0.63             00000000.00000000.00000000.00 111111
=>
Network:   172.16.4.64/26       10101100.00010000.00000100.01 000000
HostMin:   172.16.4.65          10101100.00010000.00000100.01 000001
HostMax:   172.16.4.126         10101100.00010000.00000100.01 111110
Hosts/Net: 62                    Class B, Private Internet
``````

So in your case, 172.16.4.127 is the broadcast for the 172.16.4.64/26 network. And for a complete answer:

• 172.16.4.127/26
• Network: 172.16.4.64/26
• 172.16.4.155/26
• Network: 172.16.4.128/26
• 172.16.4.193/26
• Network: 172.16.4.192/26
• HostMin: 172.16.4.193
• 172.16.4.95/27
• Network: 172.16.4.64/27