Routing Basics

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This material was developed with funding from the National Science Foundation under Grant # DUE 1601612

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Made up of millions of routing devices

Internet Assigned Numbers Authority The Internet Assigned Numbers Authority (IANA) oversees global IP address allocation to ensure that routers across the globe can forward traffic to an organization’s assigned IP network address.

What is Routing?

190.100.1.0/24

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Our Assigned Network Address Destination Network Address Full Class C = 190.100.1.0 255.255.255.0 Network Address = 190.100.1.0 Address Range 190.100.1.1-254 Broadcast Address = 190.100.1.255

Routing is the process of selecting a path for traffic in an organization’s network between or across multiple networks. Each organization must be assigned a registered IP address for routers connected to the Internet. Remember, routers forward traffic between networks.

Internet

The organization improves performance and security by subnetting the assigned network address. Subnetting requires routers. The organization’s router has four physical connections called interfaces. Each interface connects to one of the organization’s subnets. Each subnet has an assigned network address, a range of usable IP addresses, and a broadcast address. If users in one of the subnets tries to communicate with a user in a different subnet, the router forwards the traffic to the destination subnet. If a user accesses the Internet, the router forwards and receives that traffic.

Interface 1

This router subdivides the assigned network address into subnets

Destination Network Address Full Class C = 190.100.1.32 255.255.255.240 Network Address = 190.100.1.32 Address Range 190.100.1.32-46 Broadcast Address = 190.100.1.47

Creating Subnets

Interface 3

Interface 0

Network Destination Network Address Full Class C = 190.100.1.0 255.255.255.0 Network Address = 190.100.1.0 Address Range 190.100.1.1-254 Broadcast Address = 190.100.1.255

Subnet Y

Click each Subnet box

Subnet Z

Destination Network Address Full Class C = 190.100.1.16 255.255.255.240 Network Address = 190.100.1.16 Address Range 190.100.1.17-30 Broadcast Address = 190.100.1.31

Interface 2

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Destination Network Address Full Class C = 190.100.1.48 255.255.255.240 Network Address = 190.100.1.48 Address Range 190.100.1.49-62 Broadcast Address = 190.100.1.63

Subnet X

Click each Interface box to reveal the configuration

IP Address = 190.100.1.30 Mask = 255.255.255.240

IP Address = 190.100.1.254 Mask = 255.255.255.240

IP Address = 190.100.1.46 Mask = 255.255.255.240

Subnetting requires routers to interpret the IP address and subnet to determine how to deliver (or route) network traffic to the appropriate destination device. For each of its interfaces, the router applies the subnet mask to the IP address to determine the network address for each of its interfaces. The router belongs to a Class C network (190.100.1.0). Each of the router interfaces are configured to create subnets of the organization’s network. The mask size determines how large the subnets are and the interface IP address must be one of the usable addresses in the subnet. It is common practice to use the first or last usable address in the subnet range for the router interface. In this example, the organization uses the last address.

IP Address = 190.100.1.62 Mask = 255.255.255.240

Examining Router Interfaces

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Click on each Interface to populate the table

Building the Routing Table

The router uses the interface address and subnet mask to build a routing table. Click on the router to start building the table. Remember, Interface 1 has an IP address of 190.100.1.30 and a subnet mask of 255.255.255.240. This means that Interface 1 is on the 190.100.1.16 network and is using the last available address on that network. The router can extract the network address from the interface address and subnet mask. It then populates the network, subnet mask and interface number in the routing table. By building a routing table based on the network address of each interface, the router uses the table to forward traffic through the appropriate interface.

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255.255.255.240

Mask

190.100.1.48

Exit Interface

190.100.1.16

Interface 3

Destination Network

190.100.1.240

Interface 1

Interface 0

190.100.1.32

Interface 2

Interface 1 IP Address

Interface 1 Subnet Mask

Click Router to Extract Network Address

AND Operation

Interface 1 Network Address

Extracting the Network Addresses

The router uses an AND operation on the IP address and subnet mask for Interface 1 to extract the network address used in its routing table.

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Packet 3 Destination Address 190.100.1.20 255.255.255.240

Packet 2 Destination Address 190.100.1.60 255.255.255.240

Click the packet to start routing

Routing Inbound Packets

The router uses the packet’s destination IP address and subnet mask to determine the destination network. For Packet 1, the address is 190.100.1.36 with a subnet mask of 255.255.255.240 resulting in a network address of 190.100.1.32. When a match is found in the routing table, the packet is forwarded out the appropriate interface. Packet 1 gets forwarded to Interface 2 to reach the destination network, Subnet Y.

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Packet 1 Destination Address 190.100.1.36 255.255.255.240

Packet 5 Destination Address 190.100.1.40 255.255.255.240

Routing Outbound Packets

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Packet 4 Destination Address 190.100.1.243 255.255.255.240

Now let’s examine outbound traffic. The router receives the packets as they leave the internal subnets. The router examines and forwards the packet according to the routing table. If a packet’s destination address is found on the same interface it was received from, the router drops the packet because the packet was already received by the destination host on the originating subnet.

Packet 6 Destination Address 190.100.1.21 255.255.255.240

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A default route is the last option in a routing table. If a packet’s destination address does not match any destination network in the routing table, the packet takes the default route. The default route acts as catch-all for any packet that has a destination address that is not specifically in the routing table. Interface 0 is the default route.

Packet 8 Destination Address 190.100.1.250 255.255.255.240

Default Routes

Packet 7 Destination Address 190.100.1.1 255.255.255.240

0.0.0.0

Packet 9 Destination Address 234.222.2.194 255.255.255.240

0.0.0.0

Packet 10 Destination Address 190.100.1.250 255.255.255.240

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IP Address = 195.10.10.62 Mask = 255.255.255.224

Can you Build the Routing Table?

Exit Interface

Element with Audio HTML

IP Address = 195.10.10.94 Mask = 255.255.255.224

           Interface 2

IP Address = 195.10.10.130 Mask = 255.255.255.224

Fill  in the missing information.

Examine each router interface and use the IP address and subnet mask to build the routing table above.

           Interface 3

           Interface 0

IP Address = 195.10.10.126 Mask = 255.255.255.224

           Interface 1

Packet 3 Destination Address 199.100.1.26 255.255.255.248

Packet 4 Destination Address 199.10.10.40 255.255.255.248

Packet 1 Destination Address 199.100.1.42 255.255.255.248

199.100.1.32

Routing Table Challenge

255.255.255.248

Packet 6 Destination Address 199.100.1.200 255.255.255.248

Packet 5 Destination Address 199.100.1.36 255.255.255.248

Examine each packet and review the routing table. Determine the exit interface for each packet entering the router from the cloud.

199.100.1.24

Great Job!

199.100.1.48

Packet 2 Destination Address 199.100.1.50 255.255.255.248

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Drag and drop each packet to the correct exit interface

199.100.1.40

Drag and drop each packet to the correct exit interface. Examine each packet and review the routing table. Determine the exit interface for each packet entering the router from each subnet. Packets 1 and 2 enter from subnet X, packets 3 and 4 enter from subnet Y and packets 5 and 6 enter from subnet Z.

Packet 5 Destination Address 209.20.1.102 255.255.255.240

209.20.1.64

209.20.1.112

209.20.1.96

Packet 6 Destination Address 209.20.1.91 255.255.255.240

Packet 3 Destination Address 209.20.1.70 255.255.255.240

Packet 2 Destination Address 199.100.1.70 255.255.255.240

Packet 4 Destination Address 209.20.1.115 255.255.255.240

209.20.1.80

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Packet 1 Destination Address 209.20.1.16 255.255.255.240