Fastest way to change IP addresses

Matthew Seaman m.seaman at
Sat Nov 15 12:12:40 PST 2003

On Sat, Nov 15, 2003 at 01:35:52PM -0600, Jamie wrote:
>    I want to change the IP address from to
> on a FreeBSD machine as quickly as possible. Despite my efforts, I can
> only get the change to work by editing rc.conf and rebooting the machine.
> Isn't there a more elegant way?? The man page for ifconfig seems to cover
> changing IP ALIASES, but not the primary IP of an interface.
>   In order to switch to I've tried:
> 1) ifconfig de0
>   ifconfig -a then gives me:
>   fxp0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
>         inet netmask 0xffffff00 broadcast
>         ether 00:03:47:b1:d6:1c
>         media: 10BaseT/UTP status: active
>   But then I cannot ping the gateway,

Yes -- you're doing everything right in order to change the IP number.
Except that you seem confused as to whether the network interface is
de0 or fxp0 -- I assume that's just a flub in your e-mail, and not
what you've actually done...

If it isn't that, then the problem may lie within your network gear.
This may have cached the MAC address belonging to your machine as
associated with first IP number and is getting confused by the
renumbering.  If you wait for a while it should eventually sort itself
out.  How long you have to wait depends on a number of things
including the make of routers and switches -- it could be as long as
20 minutes.

One way that you might get round this problem would be to add the new
address as an alias:

    # ifconfig de0 inet netmask 0xffffffff alias

In theory you could then go on to swap the netmasks for the two
addresses, thus making the second one the primary address, and then
delete the first address.  Possibly with some sort of delay between
each operation to let verything adjust.  But no guarrantees that will
work.  You'll have to experiment.



Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP:         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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