what type of hardware is connected to the Router's Ethernet port?
If you have it connected to a Cisco Switch, you could have multiple sub-interface's on the ethernet interface, and then put the new hosts into a different subnet... You would have to enable trunking on the switch(es) and router ethernet interface...
Personally, I am not familiar with 'secondary IPs', but you MIGHT be able to add a second IP address to the Ethernet interface...
Router>Enable
Router#Configure Terminal
Router(config)#interface ethernet0
Router(config-if)#ip address 192.168.1.2 255.255.255.0
Router(config-if)#ip address 10.1.1.2 255.255.255.0 secondary
again, I am not familiar with this command (just found it in a google search)... I did read (on
CISCO's website) that you will have problems if you are using EIGRP for routing through that interface...
Can you give a more detailed description of the Topology of the network?
How many hosts are on this 'ethernet' segment, and how many are you adding.
Based upon your not wanting to change completly to class B, I have to assume that the network is using static addresses, not DHCP in the router.?
EDIT ok, that Cisco link isn't liking me... go to Cisco's homepage and search for 'secondary IP Address'