Tom,
I believe what you mean to say by "routable" and "non-routable" is that
the router has both private (RFC: 191

and public address on it.
In that case, what you're looking for is NAT.
Here's one way to do this:
1. Configure the interface that has the private address as the
"inside" interface.
2. Configure the other interface (the one that has the public address)
as the outside interface.
3. Create an ACL that identifies what "inside" addresses should be
translated to the "outside" address.
4. Assosiate that ACL with a NAT statement that causes the router to
perform the NAT
===========================
Here's a configuration EXAMPLE:
===========================
interface FastEthernet0/0
description OUTSIDE INTERFACE TO THE INTERNET
ip address 12.12.12.1 255.255.255.252 ! <-- your public address
ip access-group 101 in ! <-- ACL stops all
the "bad" stuff
no ip unreachables ! <-- a little
security here
no cdp enable
ip nat outside ! <-- THIS is
the outside
!
interface FastEthernet0/1
description INSIDE INTERFACE TO PRIVATE NETWORK
ip address 10.1.1.1 255.255.255.0
ip nat inside ! <-- THIS is
the inside
!
ip nat inside source list 1 interface FastEthernet0/0 overload
!
access-list 1 permit any
!
access-list 101 remark PREVENT UNWANTED ACCESS
access-list 101 remark DENY RFC 1918 SOURCES
access-list 101 deny ip 10.0.0.0 0.255.255.255 any
access-list 101 deny ip 172.16.0.0 0.0.15.255 any
access-list 101 deny ip 192.168.0.0 0.0.255.255 any
access-list 101 remark ANTI-SPOOFING PROTECTION
access-list 101 deny ip host 0.0.0.0 any
access-list 101 deny ip 127.0.0.0 0.255.255.255 any
access-list 101 deny ip 192.0.2.0 0.0.0.255 any
access-list 101 deny ip 224.0.0.0 31.255.255.255 any
access-list 101 remark DENY BROADCASTS
access-list 101 deny ip 255.0.0.0 0.255.255.255 any
access-list 101 deny ip any 255.0.0.0 0.255.255.255
access-list 101 remark PERMIT/DENY a few knowns
access-list 101 permit icmp any any echo-reply
access-list 101 permit icmp any any time-exceeded
access-list 101 deny icmp any any echo
access-list 101 remark PREVENT ANY INBOUND SNMP
access-list 101 deny udp any any eq snmp
access-list 101 deny udp any any eq snmptrap
access-list 101 remark ICMP TYPES
access-list 101 deny icmp any any
access-list 101 remark PREVENT CISCO CODE VULNERABILITY
access-list 101 deny 53 any any
access-list 101 deny 55 any any
access-list 101 deny 77 any any
access-list 101 deny pim any any
access-list 101 remark PERMIT everything else
access-list 101 permit ip any any
Good luck
J.Cottingim