Why W2K8 (and Vista) ignores the routing table when the destination is an APIPA address?I mean, if I have a server on my network and I try:ping 169.254.x.xW2K8 throws an ARP request instead of sending the packet to the router (as indicated by the routing table).

APIPA addresses are not routable. They do not have a default gateway setting.They are basically to allow machines on a small network to communicate without the need for a DHCP server.http://www.petri.co.il/whats_apipa.htm To reach a router by default, each workstation needs a default gateway set to the router's IP address.Bill

Surely I have not explained well (my English is too bad). If I have a server with a 'proper' IP (a non-APIPA address) and I try to comunicate with a APIPA address, the server ignores the routing table, trying to deliver the packet to the local network. For example > ipconfigWindows IP ConfigurationEthernet adapter Local Area Connection:Connection-specific DNS Suffix . :IP Address. . . . . . . . . . . . : 192.0.2.11Subnet Mask . . . . . . . . . . . : 255.255.255.0Default Gateway . . . . . . . . . : 192.0.2.1 > route print...===========================================================================Active Routes:NetworkDestinationNetmaskGatewayInterfaceMetric0.0.0.00.0.0.0192.0.2.1192.0.2.11266127.0.0.0255.0.0.0On-link127.0.0.1306127.0.0.1255.255.255.255On-link127.0.0.1306127.255.255.255255.255.255.255On-link127.0.0.1306192.0.2.0255.255.255.0On-link192.0.2.11266192.0.2.11255.255.255.255On-link192.0.2.11266192.0.2.255255.255.255.255On-link192.0.2.11266224.0.0.0240.0.0.0On-link127.0.0.1306224.0.0.0240.0.0.0On-link192.0.2.11266255.255.255.255255.255.255.255On-link127.0.0.1306255.255.255.255255.255.255.255On-link192.0.2.11266===========================================================================Persistent Routes:NetworkAddressNetmaskGatewayAddressMetric0.0.0.00.0.0.0192.0.2.1Default > ping 169.254.3.8 The server assumes that the destination computer is in the same subnet (ignoring the routing table completely) and tries to deliver the package directly (launching an ARP to find the MAC of the computer with IP 169.254.3.8) > ping 172.24.3.8 In this case the behavior is as expected. The destination is on a different network so the server delivers the package to the router

There is nothing wrong with your English. Read my first sentence again. APIPA addresses are not routable.Bill

My question is about the routing table. I understand what it is and how APIPA works.Microsoft says:'When an IP packet is to be forwarded, the routing table is used to determine: the next-hop IP address and the next-hop interface'http://technet.microsoft.com/en-us/library/dd379495.aspxI wonder why the routing table is not consulted when the destination address isone of169.254.x.x.By the way, Windows 2003 uses the routing table for every destination.

You may undersatand how APIPA works, but you do not understand how IP routing works. How do you know that the router is not forwarding the packet? Have you used a network sniffer like Netmon or Wireshark to prove this? The real point is that it would not matter whether this happened on not. That is not where your real problem is. The problem is at the server with the APIPA address. Even if it received this packet, there is no way that it could reply. A machine with an APIPA address can communicate with other APIPA machines directly (ie on the Ethernet) using the MAC address. It cannot communicate with any any other machines! It cannot route traffic.Bill

Bill, I think you are not reading carefully my posts before answering.The server has not an APIPA address. This is the key to this question.Of course I'm using a sniffer. After discovering this feature I've assembled a lab for testing purposes. Please review the output for the ipconfig and route commands.If you stop thinking (only) about APIPA and give it a chance, you can see it yourself: start a sniffer and execute ping 169.254.3.8 on your own server.Regards.

I guess that the IP stack is smart enough to know that APIPA addresses are not routable. The fact still remains that it does not really matter whether it sents the packet to the router or not. If it did reach the router, what would the router do with it?Bill

Well, this forum is a little frustrating. On 7 days only one person, who believes he knows more than anyone, has been interested in this question.@Bill: the IP stack is smart enough? Have you ever thought why there are entries in the routing table for the loopback address? I guess you don't.By the way, do you think you are helping people if you are posting without reading and replying with insulting phrases like 'you do not understand how IP routing works'?

I am sorry that you feel insulted, but the fact remains that you do not understand how IP routing works. IP routing is a two way process. It is not sufficient for a packet to reach its destination. It is also necessary that the target machine can send a reply. Even if you ever manage to get a packet from your machine to the server with an APIPA address, there is no way thet that server can send a reply to you, because it cannot route. APIPA addresses cannot be routed, just as private IP addresses cannot be routed through the public Internet. Your ping is failing at the first hurdle. If you could get past that, it would fail at the next one (and so on). Just accept that APIPA addresses are not routable and get on with your life.Bill