Under NetBSD/i386, I can do things like:

bud# ifconfig ed2 alias 205.233.54.130 netmask 0xfffffff0 
bud# netstat -rn
Routing tables

Internet:
Destination      Gateway            Flags     Refs     Use    Mtu  Interface
default          198.53.206.4       UG          4     3677      -  ppp0
127.0.0.1        127.0.0.1          UH          0        0      -  lo0
192.139.46.128   link#1             UC          0        0      -  ed2
192.139.46.129   8:0:20:0:97:c7     UHL         4     4785      -  ed2
192.139.46.130   0:0:c0:4a:72:7a    UHL         2      719      -  lo0
192.139.46.143   link#1             UHL         1       55      -  ed2
198.53.206.4     198.53.206.65      UH          1      276      -  ppp0
205.233.54.128   link#1             UC          0        0      -  ed2

but, on my Sun3 system I do:

amaterasu# ifconfig le0 alias 205.233.54.134 netmask 0xfffffff0
ifconfig: SIOCAIFADDR: Operation not permitted

I also noticed:

amaterasu# ifconfig le0
le0: flags=863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX>
        inet 192.139.46.134 netmask 0xfffffff0 broadcast 192.139.46.143
 
  The multicast flags are not there.

  Is this a thing with the le0 driver not being up to multicasting
yet? I could understand that. But not understanding aliases seems
strange, since I thought that this was handled by the machine
independant ethernet layer code.

  The reason for this is that I'm trying to see if I can make an IP
address change completely painless...
  I also haven't found any documentation on the tun interface. Is
there any written?