My recommendation is to use a completelly independant domain for your internal network. Something like mycompany.local or if you use mycompany.com and you own mycompany.net, use it, but it will block the domain for la later usage.
If you use a sub-domain (internal.mycompany.com) you will have to ask your ISP to delegate you this zone and to STRICLTY avoid have it accessible from Internet. If once for any reason anyone could resolve your internal.mycompany.com, then he will be able to have the best image any hacker could imagine of you LAN.
– use mycompany (as a top level domain) -> www.mycompany for your intranet
– use mycompany.local -> www.mycompany.local for your Intranet
Have good evening