Officially, you can use the following internally

• .test
• .example
• .invalid
• .localhost

.local is used for mDNS (multicast DNS) so don't use it (though you will probably get away with using it if you are lucky).

SAC 045 (see more info here) recommends that the list above be extended to include

Also RFC 6762.

• .local
• .localdomain
• .domain
• .host

• .lan
• .home
• .corp
• .intranet
• .internal
• .private

Project AS112 resolves NSDOMAIN for private queries on the Internet.