Question

Suppose there are three routers between a source host and a destination host. Ignoring fragmentation, an IP datagram sent from the source host to the destination host will travel over how many interfaces? How many forwarding tables will be indexed to move the datagram from the source to the destination?

Short answer

The datagram travels over 8 interfaces and 3 forwarding tables.

Step-by-step solution

Understanding Network Interfaces

Each device in a network, like routers and hosts, has network interfaces. These interfaces are points of connection to the network. Each router typically has at least two interfaces -- one for incoming data and one for outgoing data -- allowing it to connect to different network segments.

Counting Interfaces

The IP datagram will originate from the source host, which has one interface to send the datagram out. For each of the three routers, the datagram will pass through two interfaces: one coming in and one going out. Finally, the destination host will have one interface to receive the datagram. Thus, total interfaces = 1 (source) + 3(routers) x 2 + 1 (destination) = 8 interfaces.

Understanding Forwarding Tables

A forwarding table, often found in routers, indexes the incoming IP datagram's destination address to decide the next hop along its route to the destination.

Counting Forwarding Tables Indexed

For the datagram to travel from the source to the destination, the forwarding table at each of the routers will be used once to determine the next path segment. Thus, with three routers, there are three forwarding tables indexed during this journey.

Key concepts

Network Interfaces

In the world of networking, every device that communicates within a network does so through network interfaces. These are the connecting points through which data packets enter or leave a device. Think of each interface as a door – on one side is your device, and on the opposite side is the network.
For example, your computer has a network interface card (NIC) which connects it to the internet, while a router might have multiple interfaces to manage data traffic efficiently. Each router typically has at least two network interfaces:

• One for incoming data packets from the previous hop (or device).
• One for sending those packets onward to the next stop along the network path.

When data, like an IP datagram, travels from a source to a destination, it often crosses multiple network interfaces. In a setup with three routers between a source and destination host, the data will pass through multiple interfaces:

• 1 interface at the source (sending the data out).
• 2 interfaces per router where the data enters and exits.
• 1 interface at the destination (receiving the data).

Adding these up, the data goes through a total of 8 interfaces when traveling over these paths.

Forwarding Tables

Forwarding tables are vital components in a router that help make intelligent decisions about where packets should go next. When a data packet arrives at a router, the router does not randomly decide where to send it. Instead, it consults its forwarding table to make this decision.
A forwarding table contains:

• Entries that map destination addresses or prefixes to a next-hop or interface.
• Rules about handling data packets for efficient routing.

Each entry in the table tells the router which next hop to take to reach various network destinations. When an IP datagram is placed on its travel path, each router it encounters will index its forwarding table at least once to decide the upcoming segment of its journey.
In practical terms, for the datagram to reach its destination from the source, each of the three routers involved will reference its forwarding table, leading to a total of three times the datagram will be forwarded based on these tables.

Data Routing

Data routing is the process by which data packets find their way across networks from a source to a destination. It’s similar to how you use GPS directions to get from one place to another. Routers play the key role here, acting like traffic directors ensuring that data packets get where they need to go in the most efficient way possible.
Data routing involves a lot of decision-making. Routers decide the best path for data packets based on criteria like shortest path, least congestion, or fastest connection, relying on various protocols to manage these decisions:

• Dynamic routing protocols determine routes based on current network conditions.
• Static routing uses pre-defined routes set by network administrators.

During data routing, each router interprets the IP header of incoming packets, assesses the destination address, consults its forwarding table, and forwards the packet to the next hop. This method continues through successive routers until the data successfully reaches its intended destination. In our example situation with three routers, this routing process ensures the data navigates effectively through the network landscape.