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We have been assigned a block of 64 IP's by ARIN (/26). We have a private range that is a /22. Every time we get a new block of public IP's we NAT them to the next available addresses in that range, so no new private subnets being made for this new /26 block.

My question is: how many will be usable by hosts? Normally I'd say 61 (1 for network, 1 for gateway, 1 for broadcast) but with a public block not sure.

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  • to have someone properly answer this question you must include details about the amount of users/services/etc. Also, this is probably a better question for chat since it is opinion-oriented. If you are able to provide services and approx # of hosts I will gladly answer with the math that i would recommend Commented Dec 9, 2013 at 17:09
  • Those details aren't necessary at all. It's simple: 64 public IP's from ARIN, how many are usable/NATable and why?
    – A L
    Commented Dec 9, 2013 at 17:11
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    This is still worded as an opinion-based question, please revise it. Your question is akin to "i have a /24, what ip do i give my router and how many do i assign to hosts?" If your question is simply asking how many of the 64 addresses do you have the CAPABILITY to nat, that is a different question than what you have asked. Commented Dec 9, 2013 at 17:25
  • Edited for clarity, I was asking about capability. I included a bit more context as well. Thanks for your help!
    – A L
    Commented Dec 9, 2013 at 17:37
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    This still really hinges on the equipment in use and how the network is designed. This could be all of them, or it could be 61, 59, etc.
    – YLearn
    Commented Dec 9, 2013 at 18:08

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IP addresses are IP addresses are IP addresses. Until (and unless!) you end up with an assignment of IP addresses in an ethernet (or ethernet-like) network, there is no concept of "network address" "broadcast address" and the like.

NAT functions are not "an ethernet(-like) network", so all IP addresses in a block like that are usable in a NAT configuration.

There's a very strong argument to be made for not setting NAT configurations up like that, though. If you're just mapping one set of IP addresses up as 1-1 NAT mappings to another "private" (air-quotes intended) set of IP addresses, then why not just assign the public addresses directly onto the systems and eliminate the NAT altogether?

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  • Hey Jeff, thanks for a clear answer. As far as why they're not assigned directly to the boxes I couldn't say as I'm not directly involved with those decisions, I just saw the ticket on our end and the discussion along the same line and it made me wonder :)
    – A L
    Commented Dec 11, 2013 at 16:24

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