There are no direct limits on how many levels i.e. dots you can have in a hostname. However, a RFC1034 compliant hostname can only be 255 bytes long, leaving 253 bytes for a fully qualified domain name FQDN in DNS. Some systems and TLS/SSL limits FQDN to 64 bytes and FQDN in emails should not exceed(*) 245 or 221 or 189 bytes depending on the maximum user name length (8, 32 or 64).
As TLD usually takes at least 2 characters and
. and every part of the hostname must be at least one character long, the space left for additional dots i.e. theoretically maximal levels would be:
- (253-3)/2 = 125 levels after TLD for theoretically longest (not so useful) hostname
- (221-3)/2 = 109 or (189-3)/2 = 93 levels after TLD, if you wish to use it for email
- (63-3)/2 = 30 levels after TLD, if you wish to use SSL/TLS.
firstname.lastname@example.org is in a valid email address format.
(*) The special limitation for email address length is a result of RFC 5321 188.8.131.52 and 4.1.2:
The maximum total length of a user name or other local-part is 64
The maximum total length of a reverse-path or forward-path is 256
octets (including the punctuation and element separators).
4.1.2 Command Argument Syntax
Path = "<" [ A-d-l ":" ] Mailbox ">"
As forward-path must include the angle brackets, only 254 characters are left for the email address. Then, the
username@ part of 8(+1), 32(+1) or 64+1 must be excluded to get the maximum FQDN lenght.