An IPv4 host's interface shows 169.254.37.12/16 instead of its usual address; what does that address signal, and how did the host choose it?

## What the address tells you An address in **169.254.0.0/16** is an **IPv4 link-local address** (RFC 3927, Standards Track). It exists so that hosts on one link can talk to each other when there is no DHCP server and nobody configured them by hand: two laptops joined by a cable, a printer on an unmanaged segment. Seeing one on a machine that normally has a routable address is a diagnostic signal. RFC 3927 says a host should not run a link-local address alongside an operable routable one, and many host implementations self-assign a link-local address when their DHCP exchange produces no lease. So the reading is: **this interface has no working routable configuration**. Typical causes sit on the DHCP path, not in the host: - the DHCP server is down or its pool is exhausted, so no offer arrives; - the host is on a segment with no server and no working relay toward one; - the port or segment is in the wrong VLAN, so the broadcasts reach nobody; - a filter on the link drops the client's or server's messages. One host with a 169.254 address points at that host or its port; a whole segment of them points at the server or relay. ## How the host picks and claims the address RFC 3927 gives a fixed procedure: 1. **Select** an address with a pseudo-random generator, uniformly in `169.254.1.0` to `169.254.254.255` inclusive. The first 256 and last 256 addresses of the /16 (`169.254.0.x` and `169.254.255.x`) are reserved and must not be chosen. The generator should be seeded from something unique to the host, such as its MAC address, so the same host tends to pick the same address every boot; a host may also remember its last address and try it first. 2. **Probe.** After a random wait of up to `PROBE_WAIT`, send `PROBE_NUM` ARP Probes: ARP Requests whose sender IP is all zeros and whose target IP is the candidate address, spaced `PROBE_MIN` to `PROBE_MAX` apart. 3. **Detect conflicts.** If any ARP packet arrives whose sender IP is the candidate, or another host's probe targets the same candidate, the address is in use: pick a new one and start again. 4. **Announce.** Broadcast `ANNOUNCE_NUM` ARP announcements, in which sender and target IP are both the new address, so neighbours drop stale cache entries. 5. **Defend** the address for as long as it is used, because two separate links can be joined later. | Constant (RFC 3927 Section 9) | Value | |---|---| | `PROBE_WAIT` | 1 second (initial random delay) | | `PROBE_NUM` | 3 probes | | `PROBE_MIN` / `PROBE_MAX` | 1 to 2 seconds between probes | | `ANNOUNCE_WAIT` | 2 seconds before announcing | | `ANNOUNCE_NUM` / `ANNOUNCE_INTERVAL` | 2 announcements, 2 seconds apart | | `MAX_CONFLICTS` / `RATE_LIMIT_INTERVAL` | after 10 conflicts, at most one new address per 60 seconds | The rate limit exists to stop an ARP storm when something on the link answers every probe. ## What the host can and cannot reach - **Same link: yes.** It resolves neighbours with ARP and talks to them directly, including hosts that hold routable addresses on that link. - **Anything routed: no.** RFC 3927 says a packet with a 169.254 source or destination **must not** be sent to a router for forwarding, and a router that receives one **must not** forward it, whatever its routing table says and regardless of the TTL. That rule covers multicast too. - **No gateway.** The /16 has no default router, so the internet is unreachable from this address by design. ## Rules around the block - **Not for manual or DHCP use.** RFC 3927 Section 1.6 says 169.254 addresses should not be configured by hand or handed out by DHCP, because that bypasses the probing and defence rules; administrators who want their own local numbering should use RFC 1918 space. - **DHCP keeps going.** Section 1.9 says a host must not change its DHCP client's behaviour because it has a link-local address: no stopped retries, no altered timeouts. When a server answers, the host takes the routable address and drops the link-local one. - **IPv6 is different.** IPv6 requires a link-local address on every interface alongside any others; IPv4 link-local is a fallback for when routable configuration is missing, not a constant companion. Some platforms serve machine metadata on a fixed address inside this block. That is a deliberate use of link scope, and the reason 169.254.0.0/16 belongs on every list of destinations an outbound-request filter must refuse.