Let’s say you have this scenario:
R4 and R5 are connected via a serial cable. Configure IP Addressing so that R4 and R5 are in the 192.168.45.0/24. R4 should have an IP of 192.168.45.4/24. R5 should have an IP address off 192.168.45.5/24.
The catch: Do not configure an IP Address with the ip address 192.168.45.4 255.255.255.0 command directly on R4’s s1/1 interface.

Let’s have a look at the different ways we can solve this!
R4:
interface lo0
ip add 192.168.1.45.4 255.255.255.0
!
interface Serial1/1
ip unnumbered Loopback0
encapsulation ppp
clock rate 64000
R5:
interface Serial1/1
ip address 192.168.45.5 255.255.255.0
encapsulation ppp
no peer neighbor-route
R4#sh ip route | b Gateway
Gateway of last resort is not set
192.168.45.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.45.5/32 is directly connected, Serial1/1
C 192.168.45.0/24 is directly connected, Loopback0
R5#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.45.0/24 is directly connected, Serial1/1
The no peer neighbor-route is not needed on R5, but I put it there to illustrate a point. R5 doesn’t need the peer neighbor-route to R4 because it will see the 192.168.45.0/24 subnet as directly connected on its s1/1 interface. R4 does need the peer route to 192.168.45.5/32 otherwise it will forward any packets destined to that address out Lo0.
R4:
interface Serial1/1
ip address negotiated
encapsulation ppp
clock rate 64000
R5:
interface Serial1/1
ip address 192.168.45.5 255.255.255.0
encapsulation ppp
no peer neighbor-route
peer default ip address 192.168.45.4
R4#sh ip route | b Gateway
Gateway of last resort is not set
192.168.45.0/32 is subnetted, 2 subnets
C 192.168.45.5 is directly connected, Serial1/1
C 192.168.45.4 is directly connected, Serial1/1
R5#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.45.0/24 is directly connected, Serial1/1
We have used IPCP to give R4 and IP address. R4 needs the peer route, otherwise its routing table will only have a route to the 192.168.45.4/32 network. R5 doesn’t need the peer neighbor-route because that subnet is directly connected on the interface.
R4:
interface Multilink1
ip address 192.168.45.4 255.255.255.0
no peer neighbor-route
!
interface Serial1/1
no ip address
encapsulation ppp
clock rate 64000
ppp multilink
ppp multilink group 1
R5:
interface Multilink1
ip address 192.168.45.5 255.255.255.0
no peer neighbor-route
!
interface Serial1/1
no ip address
encapsulation ppp
ppp multilink
ppp multilink group 1
R4#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.45.0/24 is directly connected, Multilink1
The IP address isn’t on R4’s s1/1 interface! :)
Once again the no peer neighbor-route command is not needed, but is used to show that we don’t require a /32 to the remote end of the PPP connection.
R4:
interface Serial1/1
no ip address
encapsulation frame-relay
no keepalive
clockrate 64000
no frame-relay inverse-arp
!
interface Serial1/1.45 point-to-point
frame-relay interface-dlci 45 ppp Virtual-Template1
!
interface Virtual-Template1
ip address 192.168.45.4 255.255.255.0
no peer neighbor-route
R5:
interface Serial1/1
no ip address
encapsulation frame-relay
no keepalive
no frame-relay inverse-arp
!
interface Serial1/1.45 point-to-point
frame-relay interface-dlci 45 ppp Virtual-Template1
!
interface Virtual-Template1
ip address 192.168.45.5 255.255.255.0
no peer neighbor-route
R4#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.45.0/24 is directly connected, Virtual-Access1
R5#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.45.0/24 is directly connected, Virtual-Access1
We have disabled LMI with the no keepalive command so we can do back-to-back frame-relay. Once again we have used no peer neighbor-route it illustrate that we don’t need a specific /32 to the other end.
Cool Huh? Why the hell would you need this stuff outside the CCIE? You wouldn’t! Just configure the damn interface with 192.168.45.4/24! Since when was the CCIE about best practices and how you would do it in real life?! :)
Anybody know of any other methods?
Is that for saving the IP address ?
DHCP?