How many IP address aliases can practically be used on one physical Ethernet interface?

Fabian Keil freebsd-listen at
Tue Jan 31 07:38:13 PST 2006

tomasflyer at wrote:
> I am implementing and using a test bed simulating a huge amount of IP 
> clients, each preferable having a unique IP address. There is no, no 
> way to have an individual physical interface for each simulated
> client so I use IP aliases. Currently it runs on Linux and there is a
> limit of 256 IP addresses per interface, among other things due to a
> hard array limit in Linux net-tools ifconfig. There also seems to be
> other limitations like linear searches in net-tools as well as in
> kernel networking code. Just changing the array limit changed the
> problem to being one of stability and performance.
> So I became quite optimistic reading about Virtual Hosts and IP
> aliases in the FreeBSD handbook chapter 11.9:
> rtual-hosts.html
> "A given network interface has one "real" address, and may have any 
> number of "alias" addresses".
> So is this really true and where is the catch? Will a FreeBSD 6.0 
> accept for example 8190 IP address aliases  each on say five physical 
> Ethernet interfaces? Will IP addresses be manageable to add, list and 
> delete? And how much will networking performance degrade compared to 
> using just a few aliases?

After a short test I don't think 8190 aliases will be a problem.

root at africanqueen ~ #ifconfig re0| grep inet | wc -l
root at africanqueen ~ #ifconfig re0 -alias
root at africanqueen ~ #ifconfig re0| grep inet | wc -l
root at africanqueen ~ #ifconfig re0 alias
root at africanqueen ~ #ifconfig re0| grep inet | wc -l

I don't know if there is a performance degradation on
better hardware, but for my re0 the ftp performance
seems to be the same as with only one IP.

The only "catch" I can see is that it takes a while
to create a few thousand aliases ;-)
On my AMD Athlon(tm) XP 1900+ (1578.59-MHz 686-class CPU)
I get about five aliases per second.

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