IP addresses can’t have subdomains, you must register a domain and then add an A record for your sdN.yourdomain.tld pointing to the server IP, alternatively you can add *.yourdomain.tld that way all 3rd level domains get routed to your server unless you specify differently, or lastly, create an A record for servername.yourdomain.tld, point that to your server, then create a CNAME record for sdN.yourdomain.tld and point it to servername.yourdomain.tld.

Note that this is completely unrelated to both Networking and FreeBSD, the term you’re looking for is DNS

You can’t exactly have that, but there are workarounds, try out these services - xip[dot]io and nip[dot]io, with them you can have as many sub domains as you want.