See also NetworkingBestPractices

Use of RFC:1918 addresses for routers

This has issues such as:

 [Host A] <-> [Router B] <-> [Router C] <-> [Router D] <-> [Router E] <-> [Host F ]

Now, Router B has a route to some RFC:1918 space (perhaps Host A is even on RFC:1918 behind NAT), Router D is numbered using RFC:1918 space (differently routable than Router B). Now, if the link from Router D and router E has a smaller MTU than the rest of the network, and Router B has reverse path filtering on, then A can no longer talk to F. If A traceroutes to F then it misses seeing D as well for the same reason.

RFC:1918 is evil and should be avoided at all costs, if you must use it, use it only on the edges on networks you control.

IP Aliases

Network interfaces can be assigned more than one IP address.

Eg, to create aliases for the range -


for i in `jot 20 1` ;
  ifconfig fxp0 inet 192.168.0.$i alias ;

Linux distros:

(old way)

for i in `seq 1 20` ;
   ifconfig eth0:$i 192.168.0.$i ;

(new way)

for i in `seq 1 20` ;
  ip addr add 192.168.0.$i dev eth0 ;

This also lets you assign a range, eg:

  ip addr add dev eth0

However, old tools like route(8) and ifconfig(8) won't show you anything added using this method.


This page discusses the difference in tuning networking on various OS's: