Domain Name With An Underscore. Applications might be very RFC obedient and reject the name and will not install or work in your domain. Often referred to as a low dash an underscore looks like an elongated hyphen.
While it may be perfectly valid by RFC 2181 section 11 to have underscores in a domain name you will not find any domain type ie TLD ccTLD or gLTD that supports registering domains with underscores in you are therefore restricted to hyphens as an separator. This solution will work no matter where the underscore is in the domain name. The preferred solution is to rename hostnames FQDNs that contain underscores and replace their certificates.
The important thing to know is that.
When creating a group the number sign can be used as part of the groups name. Symbols Spaces or Underscores Numbers and hyphens are the only symbols allowed in your domain name. A sub-page or sub-folder is basically what comes after the TLD. To distinguish registration types from domain names in DNS resource records registration types use underscore prefixes to separate the components that make up a registration type.