Frequently moving a laptop between networks with different proxy settings is a hassle.

Gnome should have a facility to set different proxy settings depending on what IP address the machine gets via DCHP.

Perhaps by using the "Automatic proxy configuration URL" and pointing it to file:///home/username/.gnome/networksettings/autoproxy.pac
with rules generated by a GUI along the lines of
function FindProxyForURL(url, host){
    myIp = myIpAddress();
    if (dnsDomainIs(myIp , "localhost")){
        return "DIRECT";
    }
    else if (isInNet(myIp , "100.200.0.0", "255.255.0.0"){
        return "PROXY proxy1.example.com:3128" +
                "SOCKS socks.example.com:1080";
    }
    else{
        return "DIRECT";
    }
}