I am trying to set up an Apache server at my house which can be accessed through port 80 by my home router. I was able to do this a few months ago, and then I took the website down. When I tried to set up port forwarding again, I noticed I could not access my server from my public IP (69.162.x.x).

Looking into my router's settings, I found that it had a completely different IP listed as its public IP (100.81.x.x). From my LAN, port forwarding worked through this address (can't be a private IP (?), my router assigns with 192.168.x.x), but did not work beyond my home network.

Upon some searching on Google, I found that the address my router "thought" it had was perhaps used in double NAT purposes at an ISP level. I recieved no response when trying to contact customer support.

Am I being double NATed by my ISP, or is it some issue I have overlooked?

  • 3
    You may find this helpful, and you may also want to ask your ISP if they offer static IPs and how much they charge for it (likely~$10/mo). Another option would be a VPS and tunnelling traffic through it
    – JW0914
    Jul 23 at 13:32
  • 1
    @JW0914 Yep just as I expected they want 14 dollars for that. I can rent a VPS with internet 13 times faster at half the price, thanks for the tip!
    – matharpre
    Jul 24 at 3:33
  • "I received no response when trying to contact customer support." - time to switch to another ISP.
    – Dai
    Jul 24 at 17:09
  • 1
    "I could not access my server from my public IP" - You can't normally access other machines on the LAN via the external IP address of the router (and port forwarding) since most consumer-grade routers don't support "NAT-Loopback". (?) See: stackoverflow.com/questions/66136720/…
    – MrWhite
    Jul 24 at 17:51
  • @MrWhite I was able to do it just fine a few months ago with a Netgear R6220...
    – matharpre
    Jul 25 at 18:22

My idea is that your ISP is now transitioning to IPv6, where up to now it was using pure IPv4.

The new address 100.81.x.x is reported by the whois service as:

# ARIN WHOIS data and services are subject to the Terms of Use
# available at: https://www.arin.net/resources/registry/whois/tou/
# If you see inaccuracies in the results, please report at
# https://www.arin.net/resources/registry/whois/inaccuracy_reporting/
# Copyright 1997-2021, American Registry for Internet Numbers, Ltd.

NetRange: -
NetHandle:      NET-100-64-0-0-1
Parent:         NET100 (NET-100-0-0-0-0)
NetType:        IANA Special Use
Organization:   Internet Assigned Numbers Authority (IANA)
RegDate:        2012-03-13
Updated:        2016-04-11
Comment:        This block is used as Shared Address Space. Traffic from these addresses does not come from IANA. IANA has simply reserved these numbers in its database and does not use or operate them. We are not the source of activity you may see on logs or in e-mail records. Please refer to http://www.iana.org/abuse/ 
Comment:        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 addresses are identical on two different interfaces. 
Comment:        This block was assigned by the IETF in the Best Current Practice document, 
Comment:        RFC 6598 which can be found at: 
Comment:        http://tools.ietf.org/html/rfc6598
Ref:            https://rdap.arin.net/registry/ip/

Wikipedia describes the IPv4 shared address space as:

In order to ensure proper working of carrier-grade NAT (CGN), and, by doing so, alleviating the demand for the last remaining IPv4 addresses, a /10 size IPv4 address block was assigned by Internet Assigned Numbers Authority (IANA) to be used as shared address space.[1]

This block of addresses is specifically meant to be used by Internet service providers (or ISPs) that implement carrier-grade NAT, to connect their customer-premises equipment (CPE) to their core routers.

Conclusion: You are now behind Carrier-Grade NAT. If you wish to have a real IPv4 address, you should rent a static one from your ISP. Otherwise, the IPv4 that you find is only internal to the ISP.

  • 9
    How precisely did you conclude the ISP is transitioning to IPV6 if they are still using their subnet? There is no doubt the author is behind a CGN
    – Ramhound
    Jul 23 at 17:57
  • 7
    Use of this shared IPv4 address space is completely unrelated to enabling IPv6. It's actually possible there is a public IPv6 address yhat OP could use, or maybe no IPv6 at all.
    – Nobody
    Jul 23 at 22:05
  • 5
    It's just weird that you keep pushing your IPv6 idea here, it has nothing to do with any IPv6 issue - on the contrary it indicates a company that's clinging to IPv4 as long as they possibly can. Even with their working IPv4 setup, they can't magically get a larger address space.
    – pipe
    Jul 24 at 11:28
  • 2
    @harrymc What are we misunderstanding? Please give us a link to an IANA resource, IETF spec, or other authoritative source that establishes that use of the 100.81.x.x block is in any way an indication that the ISP is "now transitioning" to IPv6 instead of just procrastinating?
    – Dai
    Jul 24 at 17:07
  • 4
    @harrymc With respect, you haven't given a logically sound explanation for that specific point. The way I see it, an unscrupulous ISP would gladly risk downtime by switching to carrier-grade NAT if it means being able to kick-the-can on IPv6 transition for a few more years - and there are lots of unscrupulous ISPs around.
    – Dai
    Jul 24 at 17:47

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