> I'm considering throwing away my old 56k and switching to DSL, and I 
> would
> like to have a UNIX machine set up as a router for my mini-LAN. I've 
> got an
> old LC475 which has several pros (quiet, very low power consumption, 
> quite
> stable with NetBSD-softfloat),

You and I think alike.

> but one big problem: it can have (and has)
> only one network card. I know that what I want to accomplish is 
> possible
> using interface aliases, but my question is, are there any serious
> drawbacks in doing that?

Speed, I think, mostly because one port's a tighter bottleneck than two.

>  I remember having read somewhere that having a
> router with only one network interface is not secure -- is that true?

Well, does your DSL modem have a router, or is it just a modem? My 
understanding is that the modem-only solution basically exposes 
everything on the inside to the outside. If I understand this, it would 
mean that you would need externally visible IPs on all the machines on 
the inside of the modem.

I could be wrong about that, but I think that's what the folks at 
comcast were trying to tell me without actually giving away the clues 
as to what I could do about it if I were so inclined. Their story was 
that they would let their DHCP router assign up to five addresses on 
the customer side of that modem.

But if it includes a full router like the one I get from sannet here in 
Japan, then the modem's router may allow the use of the internal-only 
ranges, nat, virtual DMZ, and so forth. Depending on what you want to 
do, having the 475 sit act as an additional logical router could serve 
some useful purposes.

>  And
> while we are at it, where can I find info on how to do that with 
> NetBSD and
> ipfilter? I did a quick search but nothing interesting came up.

For starters, when you assign a numeric IP address to the interface, 
you can assign more than one, apparently.