The way I usually test my test boxes (and there may be a better way to do this), is to change (in windows) the c:\windows\system32\drivers\etc\hosts file.
Add a line in like this, let's say your website is www.john.com
and your local lan IP is 192.168.0.5
That way, when you type in your browser john.com, or www.john.com
, it will first look in the hosts file and find that entry and go there.
If you using a mac, I believe you would put an entry in /etc/hosts in the same format as the windows one.
Hope that helps.