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I recently got to know that IP segment 100.64.0.0/10 has been reserved by IANA for 'Shared Address Space'.

My question is: Can I use the IP Segment 100.64.0.0/10 as a private range in my network (like the way we are using 10.0.0.0/8)?

If yes, then do I need to upgrade my network appliances (like firewall or router) before using this, or can I use it as it is?

If no, then why not, and for what purposes this is segment reserved?

If possible, please share some document (or BCP) explaining (or talking about) this segment.

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The 100.64.0.0/10 address block is not private address space; it is shared address space. This is spelled out in RFC 6598, IANA-Reserved IPv4 Prefix for Shared Address Space (I highlighted the relevant verbiage):

  1. Introduction

IPv4 address space is nearly exhausted. However, ISPs must continue to support IPv4 growth until IPv6 is fully deployed. To that end, many ISPs will deploy a Carrier-Grade NAT (CGN) device, such as that described in [RFC6264]. Because CGNs are used on networks where public address space is expected, and currently available private address space causes operational issues when used in this context, ISPs require a new IPv4 /10 address block. This address block will be called the "Shared Address Space" and will be used to number the interfaces that connect CGN devices to Customer Premises Equipment (CPE).

Shared Address Space is similar to [RFC1918] private address space in that it is not globally routable address space and can be used by multiple pieces of equipment. However, Shared Address Space has limitations in its use that the current [RFC1918] private address space does not have. In particular, Shared Address Space can only be used in Service Provider networks or on routing equipment that is able to do address translation across router interfaces when the addresses are identical on two different interfaces.

This document requests the allocation of an IPv4 /10 address block to be used as Shared Address Space. In conversations with many ISPs, a /10 is the smallest block that will allow them to deploy CGNs on a regional basis without requiring nested CGNs. For instance, as described in [ISP-SHARED-ADDR], a /10 is sufficient to service Points of Presence in the Tokyo area.

This document details the allocation of an additional special-use IPv4 address block and updates [RFC5735].

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That block of addresses is reserved for service providers to be able to do NAT in such a way that it doesn't conflict with the normal private address space. If you start using it as private space then you're potentially creating a conflict again, so no: don't use this as private space.

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Officially RFC 6598 says

Devices MUST be capable of performing address translation when identical Shared Address Space ranges are used on two different interfaces.

Most NAT implementations are not capable of handing that case at least not witout extra hacks (on linux for example I belive that to implement NAT with overlapping internal and external space you would need to NAT the traffic twice in two different network namespaces).

You are of course free to ignore that paragraph and use the addresses anyway. RFCs are not laws. Using "shared address space" for your internal networks is certainly a lesser evil than using squat space.

If you do choose to ignore it and your ISP changes your connection to CGN then there is the risk of an addressing conflicts.

So like many things it comes down to a risk assessment. How badly do you need extra private address space? how likely is it that your internet connection will be put behind a CGN in the future?

If you are thinking about using this block because you are short of regular private addresses it's probably time for a long hard look at your IP addressing policies. Do you really have millions of devices on your internal networks? Are you wasting IPs with oversized allocations? isn't it time you thought about IPv6?

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Technically you can use that space in your network, it is just another IP subnet, but you can't advertise this network out to the public internet and any internal devices using these IPs should have a source NAT applied before that traffic enters the internet. I wouldn't recommend duplicating a public subnet though.

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My understanding of this (could be flawed) is that ISP's with a shortage of external registered IPv4 addresses may elect to put an address from this range as the OUTSIDE address of a customer's router. They then will apply a NAT further out (on a NAT appliance capable of MANY active entries) if the customer traffic needs to route outside the ISP. This means that IF you have your own /24 or more of registered external addresses that you use for ALL your external Internet addresses, your ISP's won't need to assign your CPE an address from 100.64.0.0/10 - so these addresses would all be available for you to use internally. Just don't try to route them to anyone else - even over private interconnects, as you don't know if their ISP might be using them.

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100.64.0.0 this ip belongs to class A ip address. There are some range of private ip address for each class of ip address. For class A private ip range is 10.0.0.0/8. So if you want a private ip from class A then that belongs to 10.x.x.x ip segment. You cant allocate 100.64.0.0 ip address as a private ip.

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    Network classes have been dead for over 20 years, killed in 1993 by RFCs 1517, 1518 and 1519. Please let then rest in peace. The 100.64.0.0/10 range in not publicly routable address space.
    – Ron Maupin ♦
    Commented Dec 9, 2016 at 18:18
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    That sound good Ron. But if you want to learn first think about the basic may be they are dead or alive. It does not matter.
    – Krishanu Chakraborty
    Commented Dec 10, 2016 at 4:09
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    No. Learn how to subnet, first. Then you can learn about legacy networking, e.g. classes. Don't pollute your mind with irrelevant information, first.
    – Ron Maupin ♦
    Commented Dec 10, 2016 at 4:32
  • @RonMaupin What's funny is even as recently as 2020 I've seen the "classes" still showing up in network curriculum. I'm not saying it's a good thing, just saying that Networking 101 classes for whatever reason still seem to teach the old class system (and don't even make it clear at first that it's absolutely irrelevant these days). I believe even a few certification tests still include the old classes. Some IP calculator tools still either suggest or even force you to specify the class when calculating IP address subnets. It needs to die, but it keeps lingering for some weird reason.
    – mngeek206
    Commented Jun 24, 2023 at 6:46
  • @fdmillion, the very last section of this two-part answer deals with exactly that. It is purposely put as the very last section of the answer because IP addressing and subnetting should be learned first, then you can apply historic classes to that knowledge, instead of first learning classes and having to distort that knowledge to learn proper IP addressing and subnetting.
    – Ron Maupin ♦
    Commented Jun 24, 2023 at 15:01

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