Add additional IP on existing Linux box assuming you already have eth0 NIC defined on Linux
sudo ip address add 192.168.1.125/24 dev eth0
This will add new IP address '192.168.1.125' on eth0 device. So we are listening to a new IP address on eth0 also we can add multiple other IPs to eth0 at this point.
You can see all your additional IPs on eth0 using the following command.
ip a | grep eth0
Let's assume the Linux box already had a default IP address that was accessible from the network '192.168.88.250'.
We need to define routes for new IP addresses we defined in Linux box in the other servers.
Let's assume we have a windows system that we want to communicate on the Linux server using '192.168.1.125'
On the window server we need to run the route command and define a new route as follows;
route add 192.168.1.0 MASK 255.255.255.0 192.168.86.250
This route will redirect traffic for 192.168.1.0 segment to 192.168.86.250 gateway which is our Linux server.
On the Linux box lets listen to TCP commination using Netcat (nc) command.
nc -l 192.168.1.125 3000
On the window system we can use telnet or nc command to connect the linux nc listening server.
nc 192.168.1.125 3000
There you have it!
Network Namespaces allows admins to segregate different network on the same computer. The network namespaces are isolated networks that can't directly communicate with the host system or each other!
List all network namespaces
sudo ip netns
Add new namespace
sudo ip netns add network-1
sudo ip netns add network-2
Add virtual ethernet in the namespace
This command creates pair of NIC veth0@veth1 and veth1@veth0
sudo ip -n network-1 link add type veth
List all NICs on the network namespace
sudo ip -n network-1 link
Bringing veth0 NIC up on network namespace
sudo ip -n network-1 link set veth0 up
Assigning IP to NIC in the network namespace
sudo ip -n network-1 a add 192.168.1.125/24 dev veth0