If you want to configure a network card, you have to decide on the IP-address, FQDN and possible aliases for use in the /etc/hosts file. An example is:
<my-IP> myhost.mydomain.org aliases
Make sure the IP-address is in the private network IP-address range. Valid ranges are:
Class Networks
A 10.0.0.0
B 172.16.0.0 through 172.31.0.0
C 192.168.0.0 through 192.168.255.0
A valid IP address could be 192.168.1.1. A valid FQDN for this IP could be www.linuxfromscratch.org
If you're not going to use a network card, you still need to come up with a FQDN. This is necessary for programs like Sendmail to operate correctly (in fact; Sendmail won't run when it can't determine the FQDN).
If you don't configure a network card, create a new file /etc/hosts by running:
cat > /etc/hosts << "EOF"
# Begin /etc/hosts (no network card version)
127.0.0.1 www.mydomain.com <value of HOSTNAME> localhost
# End /etc/hosts (no network card version)
EOF
If you do configure a network card, create a new file /etc/hosts containing:
cat > /etc/hosts << "EOF"
# Begin /etc/hosts (network card version)
127.0.0.1 localhost.localdomain localhost
192.168.1.1 www.mydomain.org <value of HOSTNAME>
# End /etc/hosts (network card version)
EOF
Of course, change the 192.168.1.1 and www.mydomain.org to your own liking (or requirements if you are assigned an IP-address by a network/system administrator and you plan on connecting this machine to that network).