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I'm new to networking and trying to understand how CIDR is still relevant with the introduction of NAT. If we have a public/private scheme for routers, how is CIDR still relevant? Is this at an ISP level?

Here's what I know: Classful addressing ended in the mid-90s and that CIDR replaced it. CIDR notation has suffix indicating the number of bits for the host. This allows for the host and network to be broken up differently than with the classful addressing, but I still don't see how this could be compatible in a scheme where NAT (public/private) addressing is used.

If they are different concepts, could someone kindly explain, with a home network for example how they differ?

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    CIDR and NAT are two very different subjects. Your question does not make sense. – Kevin Bowen Apr 10 '18 at 7:19
  • I think that's starting to make sense to me. Would you mind elaborating on how they are as that might answer my question? – Victor Rodriguez Apr 10 '18 at 8:26
  • "could someone kindly explain, with a home network for example" Home networking is explicitly off-topic here. You can ask about that on Super User. – Ron Maupin Apr 10 '18 at 12:35
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CIDR is relevant in that it defines how networks of various sizes, including subnetting and supernetting, can be defined in a very flexible manner. This is in contrast to the classful subnetting that was used until 1993.

The distinction of private and public IP addresses and NAT are kludges to cope with the fact that the IPv4 address range is far too small to handle the traffic of an entire planet. It contradicts the end-to-end principle of TCP/IP and produces a countless number of problems.

At the ISP level with CG NAT this becomes especially apparent when a client doesn't even get a 'real' Internet connection, just an access method.

NAT is used to interconnect private and public IP network ranges. Whether you use CIDR on either side isn't important but there's little alternative. Note the private address ranges are still compatible with classfull routing though.

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  • Thank you Zac. To clarify, how would CIDR or classful addressing be used in a LAN for example or within NAT?. In that case, what is the host and what is the network? – Victor Rodriguez Apr 10 '18 at 6:40
  • Well, you could for instance use 10.0.0.0/8 for your entire company network, with 10.0.0.0/16 to 10.255.0.0/16 being used as location-specific subnets, 10.x.0.0/24 to 10.x.255.0/24 as building/floor-specific and so on. As stated, NAT just translates public to private IP addresses or vice versa; the NAT router doesn't care about subnetting beyond its own links. – Zac67 Apr 10 '18 at 10:51

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