I have a serious question! Why APIPA alway start with 169? Why it doesn't start with 120 or 195 or something like that? What is the matmatical purpose behind it?
Thanks
3 Answers
There is no mathematical purpose; it's just a regular /16 IP address block that has been allocated by ARIN (the North American IP address registry) for this specific use.
The RFC 3927 standard (2005) that defines this use is mostly a joint work between Apple and Microsoft, but it was originally submitted in 1998 as draft-ietf-dhc-ipv4-autoconfig to act as documentation of what Windows and MacOS had already been doing at the time (that is, both Windows 98 and MacOS 8.5 already had the functionality eight years before the RFC).
So the most likely explanation (while I'm waiting for ARIN's WhoWas access) is that it was a standard IP block assignment requested either by Apple or Microsoft, both of which had implemented the Zeroconf addressing feature by themselves in ~1997 before it was standardized, and ARIN simply gave them whatever first free /16-sized block they had available.
Both companies are based in the USA, so they primarily get their IP addresses from the ranges that had been issued to ARIN, whereas from your other examples, 195.x had already been issued to the European RIPE, and 120.x had not been allocated at all yet (it was later given to APNIC).
answered May 12 at 13:06
Why APIPA always start with 169?
That is the range that is specified in RFC 3927 Dynamic Configuration of IPv4 Link-Local Addresses:
To participate in wide-area IP networking, a host needs to be configured with IP addresses for its interfaces, either manually by the user or automatically from a source on the network such as a Dynamic Host Configuration Protocol (DHCP) server. Unfortunately, such address configuration information may not always be available. It is therefore beneficial for a host to be able to depend on a useful subset of IP networking functions even when no address configuration is available. This document describes how a host may automatically configure an interface with an IPv4 address within the 169.254/16 prefix that is valid for communication with other devices connected to the same physical (or logical) link.
IPv4 Private Address Space and Filtering - American Registry for Internet Numbers:
According to standards set forth in Internet Engineering Task Force (IETF) document RFC-1918, the following IPv4 address ranges are reserved by the IANA for private internets, and are not publicly routable on the global internet:
10.0.0.0/8 IP addresses: 10.0.0.0 – 10.255.255.255
172.16.0.0/12 IP addresses: 172.16.0.0 – 172.31.255.255
192.168.0.0/16 IP addresses: 192.168.0.0 – 192.168.255.255
Note that only a portion of the “172” and the “192” address ranges are designated for private use. The remaining addresses are considered “public,” and thus are routable on the global Internet.
Use caution when setting filters to exclude these private address ranges. In some cases, Regional Internet Registries (RIRs) have issued adjacent address space to their customers and that space is in use on the global Internet.
In August 2012, ARIN began allocating “172” address space to internet service, wireless, and content providers. There have been reports from the community that many network operators are denying access to devices having IP addresses from within the entire 172 /8 range. As a result, any device with a 172.x.x.x IP address may have difficulty reaching some sites on the global Internet. The only way to solve this problem is for those operators to reconfigure their routers or firewall access controls and filter only address space from the 172.16.0.0/12 range.
answered May 12 at 13:07
There's no mathematical purpose. It was just a right-sized block of addresses that had not already been assigned to anyone or anything at the time an IETF working group was proposing IPv4 Link-Local addressing, so when that working group requested that an address block of that size be reserved for IPv4 Link-Local, the IANA arbitrarily picked that block to fulfill the request.
