I found this great tip on the email@example.com . Thanks to Nigel Friggens.
If you are a School Network Manager, ICT Lead Teacher, Computing or TIM teacher I recommend you join this Google Group!
For those teachers who are continually adjusting their internet setting between home & school.
“How to automatically switch the proxy on their laptop browsers between home and school.”
1) create a textfile "proxy.pac" and save it to the c:\drive of the laptop with the following script
function FindProxyForURL(url, host)
return "PROXY 172.31.232.250:3128";
2) The 0 in the ip address acts as a wildcard and so long as the mask is correct the whole range will be included. Obviously you will have to change the numbers to suit.
3) In IE or Firefox put the following address in the automatic configuiration script window (ie: tools.internet options.connections.lan settings) file://c:/proxy.pac