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My Windows system got an 169.xx.xx.xx IP address the other day, and I fixed the issue, but why did Microsoft chose this address as the default?

Why not 1.xx.xx.xx or 255.xx.xx.xx? Did one guy at Microsoft say

Hey, pick a number 1 to 255....who just said 169?! OK, we're going with that for our Windows default IP address.

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It's actually not always 169.xx.xx.xx there's also 192.168.xx.xx that I've seen. – KronoS Jan 27 '11 at 19:28
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169.xx.xx.xx are very specific in their use and function though and not directly related to the use of the 192.168.xx.xx address schema – Linker3000 Jan 27 '11 at 19:32
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Yeah, no, KronoS -- you've never seen Windows automatically assign itself a 192.168 IP address... it doesn't do that. – delfuego Jan 28 '11 at 2:35
@KronoS - could it be that your router assigned that number to your computer but couldn't obtain an address from the modem, thus not giving you access to the internet? – A.Donahue Jan 28 '11 at 16:18
@delfuego you need to put an "@" in front of the name of the person you're trying to respond to... – KronoS Jan 28 '11 at 16:21
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3 Answers

up vote 40 down vote accepted

It's not MS it is the ISOC ;-)

Have a look at reserved IP address RFC 5735 under special use IPv4: here

169.254.0.0/16 - This is the "link local" block. As described in [RFC3927], it is allocated for communication between hosts on a single link. Hosts obtain these addresses by auto-configuration, such as when a DHCP server cannot be found.

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I first came across this address space when Apple introduced Bonjour. – Adam Backstrom Jan 28 '11 at 2:38
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That only defers the description of “why”, though: why did RFC 5735 allocate this specific “magic number” for this use? What’s the rationale? – Konrad Rudolph Jan 28 '11 at 14:13
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@Konrad - I suspect part of the rationale was that it was available. – DJ Pon3 Jan 28 '11 at 15:17
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@Konrad - RFC 5735 is more of a collection of RFC's that pertain to special use netblocks. The actual "meat and potatoes" if you will can be found in RFC 3927 which is linked directly from 5735. As for why the IETF picked this address space for Link local? I can't find the reasoning. – Dan M. Jan 28 '11 at 15:47
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up vote 29 down vote

The use of 169.x.x.x addresses are defined within a standard colloquially known as APIPA - Automatic Private IP Addressing.

In a nutshell, if a network device has not been assigned a fixed (static) address and cannot obtain one by asking (DHCP), the device says to itself, "Well, I'd better make up an address of my own so I can communicate on this network", so it assigns itself an APIPA address, which start at 169.254.0.1 and run up to 169.254.255.254.

If you suddenly find your computer has an address within the AIPIA range it usually means that the device on the network dishing out addresses (the DHCP server) is not contactable for some reason; it may be switched off or your network cable has become disconnected, for example.

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up vote 6 down vote

To state Dave M's answer in another terms, your DHCP server has a problem and cannot allocate an IP address. When Windows and any other OS is configured to get an IP via DHCP and they don't get any, they automatically assign 169.254.xxx.xxx IP

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