Computer Networks - Top Down approach - Network Layer

IPv4 addresses are 32-bit or 4-byte long. They are usually written in dotted-decimal notation as set of four integers between 0 and 255 (both inclusive) separated by dots. Example IP addresses are: 1.1.1.1, 0.0.0.0, 10.5.1.222, 255.255.255.255, 255.255.255.0, etc.

Netmasks are used to group several set of IP addresses together. When converted to binary form netmasks would have all leading bits 1 followed by all 0s. Hence there are 33 possible netmasks (not all of them are practically useful). Example netmasks are 255.0.0.0, 255.254.0.0, 0.0.0.0, 255.255.255.255, etc.

Netmasks are also written using slash() notation. In slash() notation we write slash(/) followed by number of 1-bits in netmask. For example 255.0.0.0 can also be written as /8. Similarly 255.255.255.255 can also be written as /32.

When bits in IPv4 are ANDed with bits in netmask then we get network number or network address of current host. All hosts which have same network number are in same network and do not need a intermediate device (router / L3 switch / Gateway) for them to communicate with each other.

For example consider two addresses 10.5.1.222/24 and 10.5.1.137/24. When we calculate network numbers for both of these addresses we will get 10.5.1.0 (or to be more precise 10.5.1.0/24). Since both addresses have same network number, they belong to same network or to be more precise, they belong to same subnet and hence can communicate with each other directly without having any intermediate L3 device.

Another example can be 10.3.3.187/24 and 10.5.1.222/24. In this case we get two different network numbers 10.3.3.0/24 and 10.5.1.0/24. Hence machine with given set of IP addresses and corresponding netmasks cannot talk with each other directly and need an intermediate L3 device usually referred to as gateway or router.

It should be noted that netmasks play crucial role in determining which machines are in same network and which are in different network. For example if we change netmask from /24 to /8 in earlier example then both IP addresses would belong to network 10.0.0.0/8 and hence would be able to communicate directly without requiring any intermediate device.

Historically only /8, /16 and /24, etc. were allowed to be used as netmasks and intermediate values like /23 and /12 were not allowed / supported. That form of addressing was known as classful addressing. Following different classes existed at time of classful addressing -

But this caused very rapid depletion of IP addresses as there was huge wastage of IP addresses. Hence CIDR or Class-less Inter-Domain Routing (pronounced sider) was introduced which allowed greater flexibility in netmasks.

Now given that two machines with IP addresses like 10.3.3.187/24 and 10.5.1.222/24 cannot talk with each other directly without going through a intermediate L3 device, how do we make machines in different networks communicate? Also what is special about this gateway device that it can communicate with both machines although they are in different networks?

In order for gateway to be able to communicate with both machines gateway device has multiple interfaces (or multiple virtual interfaces) each having a different IP address. Now one of the interfaces of gateway device (say 10.3.3.1/24) can be in 10.3.3.0/24 network while other interface (say 10.5.1.1/24) can be in 10.5.1.0/24 network. In this setup gateway device would be able to communicate with both 10.3.3.187/24 and 10.5.1.222/24 directly.

Even more now 10.5.1.222/24 and 10.3.3.187/24 can send packets to each other via 10.5.1.1/24 and 10.3.3.1/24 respectively. Gateway device an internally forward packets from 10.3.3.1/24 to 10.5.1.1/24 and vice-versa as it is one single physical device with common RAM, processor etc. and does not requires any intermediate device for this transfer.

In practical scenarios we have L3 switches (or routers) which have many interfaces and not just two or three. For example Main IIIT core L3 switches are capable of having more than 1000 virtual interfaces and hence can be used to connect more than 1000 separate networks with each other. This is just theoretical number. Due to practical limitations we do not cross limit of 64 separate networks within campus. Even today at time of this writing there are 3000+ network ports in IIIT Hyderabad divided among approx 40 different networks.

We could always have just one big network with all devices in one single large subnet. But due to broadcasts such network would always be heavily congested. Having so many devices in single network would also cause serious security problems as many networks attacks are possible or highly facilitated when both attacker and the target are in same subnet. Thus having one large network would cause serious security problems for critical infrastructure like servers, network devices, etc.

Connecting many different networks using one L3 switch / router should now be clear. But how do we connect large number of machines (over million machines over Internet) with each other. There is no single device that is powerful enough to route traffic between a million nodes. To solve this problem we have a large number of L3 switches and routers connected to each other which help in sending packet from one node to another.

Explain static routing used in IIIT Hyderabad in terms of Nilgiri building, Vindhya building and OBH (Palash nivas).

One can check current IP address(s) assigned on machine using:

/sbin/ifconfig 

command.

We can use ifconfig command to configure IP address and netmask of eth0 etc. interfaces. Syntax of ifconfig command is:

ifconfig <interface> <ipaddress> netmask <netmask>

Example, ifconfig eth0 10.3.4.2 netmask 255.255.255.0

To check current gateway (in fact entire routing table) we can use

route -n

To configure gateway or default route, when there is no gateway configuration at present, we can use:

route add default gw <gateway_ip>

Example, route add default gw 10.3.4.1

To add specific routes for a given destination / mask combination via a gateway device we can use:

route add -net <destination_network> netmask <mask> gw <gateway_address>

Example, route add -net 10.6.0.0 netmask 255.255.0.0 gw 10.4.2.208

IP address and default gateway are configured using files in /etc/sysconfig/network-scripts/ folder with name ifcfg-<interface>. For example configuration file for eth0 would have name ifcfg-eth0.

Permanent route configuration is done in file with name route-<interface> for example permanent route configuration related to eth0 would be done in file route-eth0 located in same folder (/etc/sysconfig/network-scripts). Sample route-eth0 file may look like

GATEWAY0=10.3.3.1
NETMASK0=255.255.255.0
ADDRESS0=192.168.1.0