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Ok, so I am working to get some things accomplished here at our call center (tons of details I can provide if needed).

We are getting our phone people to provide SIP to us and he is asking me to provide “two private /29 subnets”. I am thinking that his terminology and mine are not matching up here (most likely mine) because what I sent him was not what he was looking for. I tried to get clarification and I got back “any private IP space, as long as it is unique to their routing table, and will not be used in the future”.

Can you give me an example of what he is looking for?

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172.16.1.0/29

172.16.1.8/29

How’s that? Two subnets with 5 usable IP’s. Presumably you just need to put 172.16.1.1 and 172.16.1.9 on your router(s) internal interface(s) and go from there.

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You need a subnet mask of 255.255.255.248 on each network

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That’s a subnet with a mask of 255.255.255.248

255 = 11111111

255 = 11111111

255 = 11111111

248 = 11111000

29 1’s and 3 zeros are the subnet mask. That’s /29

Subnet mask tells the router “If between the source and destination there’s a difference in these bits, send it to another network”… Or something like this.

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Here’s some more help with the nomenclature. The /29 means that there are 29 bits set to one in the subnet mask. The remaining three bits are set to zero

So, for example, /24 is 255.255.255.0 (each of the 255s in decimal corresponds to 8 bits in binary: 11111111). It’s just a shorthand. The bits that are set to one are always contiguous, from most significant (left-most) to least significant. So /29 carves out the 5 most significant bits in the next byte in addition to the 24 in the previous example, producing a subnet mask of 255.255.255.248 (in decimal notation)

When you put it all together, 172.16.1.0 /29 means that the subnet spans 172.16.1.0 through 172.16.1.7. Eight addresses in total. Two of those aren’t generally available for use: one for the gateway (to route traffic into and out of the subnet) and one that’s used for broadcasts within the subnet.

Hope that helps

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Just to add to the other answers: That is called a CIDR range.

You will find a bunch of details with your search engine of choice, there are also a few good subnet calculators online that may help you.

Edit: Made a mistake, corrected in next post. Sorry.

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Ok, so it does seem that I was off, and just did not understand it. Putting all this together…I think I have it now (mostly at least).

So if I give him

172.16.2.0/29

172.16.2.8/29

That will allow them to use 172.16.2.0-15, right?

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meh, time to go to bed. /29 is 8 adresses, sorry…

So yes, but these are two subnets, not a single one.

  • 172.16.2.1-6, net is 0 and broadcast is 7
  • 172.16.2.9-14, net is 8 and broadcast is 15
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I still remember the way my QA Cisco teacher taught be how to do my “long hand” calculations. She would ask for “Barry Manilow’s Third”. - You’re the First, My Last, My Everything - IE The First, Last & Broadcast address.

Examples have already been given of a valid private /29 range so nothing to add there.

What I will say is if you want to understand subnetting better, don’t obsess over writing down all the bits, or classful vs classless. You don’t need to do any of that to subnetting quickly. It’s only difficult because people keep inventing difficult ways to do it.

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No.
It will allow a range of IP addresses from 172.16.2.1 - 172.16.2.6 for the first subnet. Which means on your end you will likely put 172.16.2.1/29…

For the 2nd subnet, you have a range of 172.16.2.9 - 172.16.2.14 for useable host or interface IP addresses. Meaning you’ll likely put 172.16.2.9/29 on your end…

Remember broadcast addresses are not useable. So 172.16.2.7/29 and 172.16.2.15/29 are NOT used

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I should have been more clear. I do understand that not all of the addresses would be actually usable/assignable, but in my head they are marked as used (therefore usable for their purposes).

Thanks for the help!

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I like subnet-calculator. I find it very helpful for clarifying/understanding subnetting. For example, if you need to find the wildcard mask for a subnet.

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Used exactly that calculator earlier this day to build a few new subnets :slight_smile:

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