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:
# available at: https://www.arin.net/resources/registry/whois/tou/
# If you see inaccuracies in the results, please report at
# Copyright 1997-2021, American Registry for Internet Numbers, Ltd.
NetRange: 100.64.0.0 - 100.127.255.255
Parent: NET100 (NET-100-0-0-0-0)
NetType: IANA Special Use
Organization: Internet Assigned Numbers Authority (IANA)
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:
Wikipedia describes the
IPv4 shared address space
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
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.