On the other hand, why not simply use subdomains of an actual domainname you own?
I do realize it's inconceivable, but some people do not own domain names. Well, I do, but they don't really match my internal naming scheme. So, my internal domain is something that wasn't valid until they came up with the stupid gTLD concept: shark species as hostname, domain "sharks" on my network and in a similar vein Kiplings Jungle Book characters as hostnames and "jungle" as domain for my parents network. This works fine, looks pretty and works.
Now of course, I could use jawtheshark.com for my internal network. As a direct consequence, I'd have to either slave my LAN DNS to a public DNS and expose my internal IP numbering to the world, or keep my LAN DNS manually synchronized with my global DNS. You see, all kind of problems I didn't have because my internal domain was completely not used on the Internet. For my parents network, I don't even have a domain name that would match the naming scheme. My dad has our surname.lu, but that hardly will match the jungle naming scheme. Well, I could just buy yet another domain name and use it only internally, but that's added cost I didn't use to have.
The gTLD stuff is just stupid. That's my opinion.