A standard network interface will look similar to the following in /etc/network/interfaces:
auto eth0
iface eth0 inet static
address 192.168.210.100
netmask 255.255.255.0
broadcast 192.168.210.255
gateway 192.168.210.1
Adding more IP addresses to those interfaces is as simple as creating an alias, done by duplicating the information for the initial interface and appending a colon followed by 0,1,2,3,etc according to how many aliases are needed, minus the gateway (the initial IPs gateway will be used):
auto eth0:0
iface eth0:0 inet static
address 192.168.210.101
netmask 255.255.255.0
broadcast 192.168.210.255
auto eth0:1
iface eth0:1 inet static
address 192.168.210.102
netmask 255.255.255.0
broadcast 192.168.210.255
When finished, run an ifconfig to verify that the new interfaces are up and then ping them from a client system.