How to changue MTU Value Permanently??

[Hueznar]

I need to changue MTU Value from 1500 to 1450 on my ethernet. I do this:

ifconfig eth0 mtu 1450.

When I type /etc/init.d/network restart after MTU Changue, I discover some IP adresses from my system are deleted (Some network interfaces dissapear), So I need to reboot my server, and again I have MTU Value to 1500. I need to know how to fix permanently MTU Value and not lost it in the reboot process.

and, also, I need to Know if exists some secure method to apply inmediately a MTU Value Changue.

Thanks for your help.