Sometimes, we do need to manually lookup the IP address of a domain name. You may already be familiar with the nslookup command which is now deprecated. We use the dig command to make DNS queries.
$ dig +noall +answer www.gnu.org
www.gnu.org. 67 IN CNAME gnu.org.
gnu.org. 67 IN A 126.96.36.199
The IP address is displayed in the A record, and is 188.8.131.52.
The +noall, +answer combination basically tells dig to only report the answer of the DNS query and skip the rest of the output.
You can also use the dig command with the -x option to do a reverse DNS lookup. A reverse DNS lookup means you want to look up the domain and host name of an IP address.
$ dig +noall +answer -x 184.108.40.206
10.41.232.199.in-addr.arpa. 36000 IN CNAME rev-c41-10.gnu.org.
rev-c41-10.gnu.org. 300 IN PTR www.gnu.org.
The PTR record is the one that contains the domain host name. The domain name is, as you expect, www.gnu.org.
Note that PTR records are not required for IP addresses. If a PTR record is not defined for an IP address, you cannot do a remote DNS lookup.