Q:

IPv4 Segment 100.64.0.0/10

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.

A:

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):

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].

A:

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.

A:

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?

