Path: utzoo!attcan!uunet!husc6!cmcl2!rutgers!apple!bionet!agate!ucbvax!ATHENA.MIT.EDU!mar From: mar@ATHENA.MIT.EDU Newsgroups: comp.protocols.tcp-ip Subject: Dynamic IP address assignment Message-ID: <8811221804.AA20970@TOTO.MIT.EDU> Date: 22 Nov 88 18:04:57 GMT References: Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 33 From: agate!bionet!lear@ucbvax.Berkeley.EDU (Eliot Lear) Organization: Natl Computer Resource for Mol. Biology Wouldn't you run into a problem with ARP caches if you dynamically assign internet addresses to ethernet based machines? Eliot Possibly, but probably not. If you dyanically assign a machine to an address that either hasn't been used before, or hasn't been used in a long enough time that everyone's ARP cache has timed out, you have no problem. If a machine is assigned an IP address for which ARP cache entries do exist on the local ethernet pointing to a different ethernet address, you still might not lose. Since you've just assigned this dynamic address, this new machine probably has an empty ARP cache which it will need entries in before it can talk to any other hosts. In the process of sending ARP requests for other hosts, those other hosts will get the new mapping for this host. So the only problem is if a machine which has a stale ARP entry attempts to contact this new machine, rather than the new machine contacting it. A way to avoid this entirely is to have the new machine broadcast an ARP reply when it starts up. This gratuitous ARP reply will prime everyone's cache with its information. Some operating systems do this (broadcast) already, and every ARP implementation following the RFC will get its cache primed. -Mark