Would comps be able to communicate with ip assigned by APIPA

[dsr_deep1]

Hi there,

Lets say DHCP server is down and all the systems in lan got ip using APIPA which ranges between 169.254.0.0 to 169.254.255.255. These systems are connected through switch, now would they be able to communicate as they are in same network range or not, if not why? PLZ help. Would the switch be able to get mac addresses using ARP and maintain mac address table for communication in this situation. THANKS in advance.

[Peter Paluch]

Hello,

If the computers are in the same LAN or VLAN, they will be able to communicate with the APIPA-assigned addresses. In fact, that is the reason that the APIPA even exists - to allow computers to communicate in a single LAN without requiring any manual IP configuration in the case DHCP is not available.

The only problem is the name-to-IP resolution. Usually, DNS is used for that but in a single LAN with APIPA addressing, the DNS would not be available. So you would either need to use IP addresses instead of computer names to communicate, or some other dynamic mechanisms would need to be used that allow stations to discover their names and IP addresses dynamically. Traditionally, Windows File and Printer Sharing services have such mechanisms. Other network services may not support it so it strongly depends on the kind of service you would like to use in an APIPA-addressed network whether you could use names or IP addresses when speaking to a different computer.

Please note that a switch never learns MAC addresses of connected stations into its MAC address table using ARP. This is a common misconception. A switch learns MAC addresses simply by observing the frames flowing through it.

Best regards,

Peter

[dsr_deep1]

Thanks for your quick reply, you been of much help. i have one last question, plz help. lets says dhcp goes down after all the clients have aquired ip address, by default dhcp leases out ips for 8 days if am not wrong, as soon as dhcp goes down would the comps continue to work with ips assigned by dhcp for 8 days or immediatley apipa assigns ip.

Is APIPA which works centrally per lan or vlan or per system, if it is system feature how does other system know that a particular ip is not assigned already. PLZ help, Thanks.

[cadet alain]

Hi,

look at this article explaining how the clients try to renew their IP addresses.

http://layer3.wordpress.com/2010/01/04/dhcppart2/

Regards

Alain

Don't forget to rate helpful posts.

[Peter Paluch]

Hello,

You are welcome.

Regarding the length of a DHCP address lease - there is no standard default. Different DHCP implementations may use different lease times, and depending on the network requirements, an administrator may decrease the lease time down to the order of minutes.

In any case, when the DHCP server goes down, clients will not notice until they try to refresh their leases. They will continue to use their DHCP-assigned addresses. Only when these addresses start to expire and clients try to refresh their leases, no DHCP server will respond. This will cause clients to time out waiting for the DHCP server's reply, and when they decide after repeated unsuccessful attempts that the DHCP server is unreachable, they will move to the APIPA-configured addresses. This process is not fast - it can take a minute or two because of the relatively gratious timeouts in DHCP clients.

Best regards,

Peter