Xen host and guest sharing same IP

To achieve this assign the public ip to host system. Then forward the required port from host system to guest system.

Suppose your guest is a web server then you need to forward port 80 from host to port 80 of guest so that every request for port 80 on public ip will be forwarded to port 80 of guest.

Suppose you have assigned public ip to eth0 and your guest's ip is 192.168.1.2, then you have to tell host that it should do NAT/MASQUERADE so that the virtual machines have internet access. You also have to tell host which ports it should forward to which IP address. You can refer following script.

#!/bin/sh

GUEST_IP=192.168.1.2

echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

### Port Forwarding ###

### web server 
iptables -A PREROUTING -t nat -p tcp -i eth0 --dport 80 -j DNAT --to $GUEST_IP:80

### mail server 
iptables -A PREROUTING -t nat -p tcp -i eth0 --dport 25 -j DNAT --to $GUEST_IP:25
iptables -A PREROUTING -t nat -p tcp -i eth0 --dport 143 -j DNAT --to $GUEST_IP:143
iptables -A PREROUTING -t nat -p tcp -i eth0 --dport 110 -j DNAT --to $GUEST_IP:110

The first two commands enable Nat/Masquerad'ing on host

### Port Forwarding ###

Here you put as many rules as you need. This tells host to forward certain ports to certain destination ports on certain destination IP addresses.

You should think about the virtualized machine as about real machine. You shouldn't have two machines with the same IP in your network. But you may use some bridge, NAT, port forwarding, if your goal is to hide the presence of virtualized environment.