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).

[https://www.icann.org/en/system/files/files/sac-045-en.pdf|SAC 045] (see more info [https://tools.ietf.org/id/draft-chapin-rfc2606bis-00.html||here]) recommends that the list above be extended to include

• .local
• .localdomain
• .domain
• .lan
• .home
• .host
• .corp