What are the rules for registering a domain name?

Your domain name must start and end with either a letter or a number. Your domain name may not exceed 63 characters. excluding the .com, .net or .org extension at the end. Hyphens are the only acceptable non-letter/number character to include in your domain name. Characters, such as exclamation signs and underscores, are not permitted. Domain is always in a lower case letters.