Glossary: MongoDB

Host addresses (127.0.0.1, 0.0.0.0)

Also written: 127.0.0.1, 0.0.0.0, 0.0.0.0/0

127.0.0.1 is your own computer, which is where mongod listens by default. 0.0.0.0/0 means every address on the internet, so keep it out of an Atlas IP access list except for a quick test.