/etc/hosts, /etc/hosts.allow, /etc/hosts.deny

/etc/hosts

The /etc/hosts file acts as a local database to provide information for DNS resolutions, in the form of mapping hostnames (or fully qualified domain names - FQDN) to IP address values.

The following is the general format for each line in the file

[IP_addr_to_resolve] [hostname_1] [optional_hostname_2]

Eg. Mapping hostname jarrettgxz.com to the IP address value of 8.8.8.8

Multiple hostnames can be included for each IP address value

• The second line maps the hostnames example.com and example2.com to the IP address 10.10.10.10.

8.8.8.8 jarrettgxz.com
10.10.10.10 example1.com example2.com 

/etc/hosts.allow and /etc/hosts.deny

The /etc/hosts.allow and /etc/hosts.deny files are used to allow or restrict access to local services for specific IP addresses or hostnames.

General format of entry

service: host/network

Note that /etc/hosts.allow takes precedence over the /etc/hosts.deny file

Eg. Allow or deny access all traffic to the sshd service

sshd: ALL

Eg. Allow or deny access to all traffic from *.example.com: test.example.com, 1.example.com, etc. to the sshd service

sshd: .example.com 

Eg. Allow or deny access to all traffic from 192.168.1.*: 192.168.1.1, 192.168.1.88, etc. to the sshd service

sshd: 192.168.1. 

hosts.allow format and example on LinuxLinuxConfig