Routing Basics

What is Routing?
How Routing Works
Routing Table
Default Gateway
Key Concepts & Terminology
Hands-On Example
Exercises

1. What is Routing?

Routing is the process of selecting a path for traffic across one or more networks. In simpler terms — it's how data finds its way from one device to another across the internet or any interconnected network.

Analogy: The Postal System

A real-world comparison to understand how routing works in everyday terms.

Think of routing like the postal system:

Network Concept	Postal Analogy
Packet	A letter/envelope
Source IP	Return address
Destination IP	Delivery address
Router	Post office sorting facility
Routing Table	Postal route map
Default Gateway	Your local post office drop-off

Just as a letter passes through multiple post offices to reach its destination, data packets hop through multiple routers to reach their target.

Routers vs. Switches

Key differences between Layer 3 routers and Layer 2 switches in a network.

Feature	Router	Switch
Layer	Layer 3 (Network)	Layer 2 (Data Link)
Decision Basis	IP Addresses	MAC Addresses
Connects	Different networks	Devices within same network
Key Feature	Routing / NAT	MAC learning / Forwarding

2. How Routing Works

Step-by-Step Process

How a packet travels from source to destination through multiple routers.

[PC A] ---> [Router 1] ---> [Router 2] ---> [Router 3] ---> [PC B]
 10.0.0.5     10.0.0.1      172.16.0.1      192.168.0.1     192.168.0.5
              +----------+  +----------+  +----------+
              |  ROUTER  |  |  ROUTER  |  |  ROUTER  |
              |   TABLE  |  |   TABLE  |  |   TABLE  |
              |10.0.0.0/24|  |172.16.0.0|  |192.168.0.0|
              |172.16.0.0|  |192.168.0.0|  |10.0.0.0/24|
              +----------+  +----------+  +----------+

The Packet Journey

PC A (10.0.0.5) wants to send data to PC B (192.168.0.5)
PC A checks — is the destination on my local network? No (different subnet)
PC A forwards the packet to its Default Gateway — Router 1 (10.0.0.1)
Router 1 checks its routing table -> finds path to 192.168.0.0/24 -> forwards to Router 2
Router 2 checks its routing table -> forwards to Router 3
Router 3 sees 192.168.0.5 is directly connected -> delivers to PC B

Key Routing Principle

Each router makes an independent forwarding decision based only on its own routing table. It doesn't know the full path — just the next hop.

3. Routing Table

A database in a router that stores known network paths and next-hop information.

What is a Routing Table?

A routing table is a data structure stored in a router (or a host) that lists the routes to particular network destinations. Think of it as a map the router uses to decide where to send packets.

Anatomy of a Routing Table Entry

+--------------+--------------+--------------+--------------+
| Destination  |  Next Hop    |   Interface  |    Metric    |
|   Network    |    (IP)      |              |              |
+--------------+--------------+--------------+--------------+
| 0.0.0.0/0    | 10.0.0.1     | eth0         | 0            |
| 10.0.0.0/24  | Direct       | eth0         | 0            |
| 172.16.0.0/24| 10.0.0.2     | eth0         | 1            |
| 192.168.0.0/24| 10.0.0.3    | eth0         | 2            |
| 192.168.1.0/24| Direct      | eth1         | 0            |
+--------------+--------------+--------------+--------------+

Field Breakdown

Explanation of each column in a routing table entry.

Field	Description	Example
Destination Network	The target network (IP + subnet mask)	`192.168.1.0/24`
Next Hop	The IP address of the next router to forward to	`10.0.0.2`
Interface	Which port to send the packet out of	`eth0`, `eth1`
Metric / Cost	How far/expensive this route is (lower = better)	`0` (direct), `1` (one hop)
Administrative Distance	Trustworthiness of the route source	`0` (connected), `1` (static), `120` (RIP)

Types of Routes

Routing entries are classified as directly connected, static, or dynamic.

1. Directly Connected Routes

Added automatically when an interface has an IP configured
Metric = 0 (best possible)
Example: 192.168.1.0/24 is directly connected, eth1

2. Static Routes

Manually configured by a network administrator
Fixed, predictable, no overhead
Best for small/stable networks

3. Dynamic Routes

Learned automatically via routing protocols (RIP, OSPF, EIGRP)
Adapt to network changes
Essential for large/complex networks

Routing Decision Logic

Step-by-step process a router follows when deciding where to forward a packet.

+----------------------------------+
|   Packet arrives with dest IP    |
|          192.168.1.50            |
+------------+---------------------+
             |
             v
    Does routing table have        --NO--> Use Default Route (0.0.0.0/0)
    a matching entry?                         |
             |YES                          [Drop / Forward to ISP]
             |
    +------------------+
    |  Longest Prefix   |
    |  Match Rule       |
    +------------------+
             |
    More specific = better match
             |
         Example:
  192.168.0.0/16  -> matches, but /24 is better
  192.168.1.0/24  -> matches, most specific
  192.168.1.128/25-> does NOT match
             |
             v
    Forward packet to Next Hop

Longest Prefix Match Rule

When multiple routes match, the most specific network prefix is selected.

When multiple routes match, the most specific (smallest network) wins:

Destination	Prefix Length	Match Level
`0.0.0.0/0`	0 bits	Default (last resort)
`192.168.0.0/16`	16 bits	General match
`192.168.1.0/24`	24 bits	Most specific

4. Default Gateway

What is a Default Gateway?

The default gateway is the router that a device uses when it needs to send traffic outside its own network. It's the "exit door" to the rest of the world.

Where It Lives

+----------------------------------------------------+
|                    HOME NETWORK                     |
|                                                     |
|  +------+    +------+    +------+                   |
|  | PC 1 |    | PC 2 |    | PC 3 |                   |
|  |.2    |    |.3    |    |.4    |                   |
|  +--+---+    +--+---+    +--+---+                   |
|     |           |           |                       |
|  +--+-----------+-----------+--+                    |
|  |       SWITCH                |                    |
|  +----------+-----------------+                     |
|             |                                       |
|     +-------+--------+       Network: 192.168.1.0/24|
|     |  DEFAULT GW    |         GW IP: 192.168.1.1   |
|     |  (Router) .1   |                              |
|     +-------+--------+                              |
+-------------+--------------------------------------+
              |
      +-------+--------+
      |   INTERNET     |
      +----------------+

How a Device Uses Its Default Gateway

+---------------------------------------------------------+
| 1. PC wants to reach 8.8.8.8                            |
| 2. PC checks: Is 8.8.8.8 on my network (192.168.1.0/24)?|
| 3. NO -> sends packet to Default Gateway 192.168.1.1    |
| 4. Router at 192.168.1.1 receives and forwards onward   |
+---------------------------------------------------------+

Checking Your Default Gateway

Windows

ipconfig

Look for Default Gateway under your network adapter.

Linux / macOS

ip route | grep default
# or
route -n

Example Output

default via 192.168.1.1 dev wlan0

The Default Route in Routing Tables

The default gateway appears in routing tables as the default route:

0.0.0.0/0 via 192.168.1.1 dev eth0

0.0.0.0/0 means "any destination"
It's the route of last resort — used when no more specific route matches

Why Default Gateway Matters

Every device needs one to reach the internet
If misconfigured, the device can talk locally but not externally
DHCP (Dynamic Host Configuration Protocol) usually provides it automatically
Can be manually set in network settings (static configuration)

5. Key Concepts & Terminology

Term	Definition
Hop	A passage through one router
Next Hop	The immediate router a packet is sent to
Metric	A value used to compare routes (fewer hops, faster links, etc.)
Administrative Distance (AD)	Trust level of a route source (0 = most trusted, 255 = unreachable)
Connected Route	A network directly attached to a router's interface
Static Route	A manually configured route
Dynamic Route	A route learned via a routing protocol
Route Summarization	Combining multiple networks into one entry (CIDR)
Floating Static Route	A backup static route with higher AD, used if the primary fails

Common AD Values

Route Source	AD
Directly Connected	0
Static Route	1
EIGRP Summary	5
External BGP	20
Internal EIGRP	90
OSPF	110
RIP	120
External EIGRP	170
Unknown / Unreliable	255

6. Hands-On Example: Building a Simple Routing Table

Network Topology

        +----------+                    +----------+
        |  Router  |   172.16.0.0/30    |  Router  |
        |    R1    |<------------------>|    R2    |
        |          |  .1            .2  |          |
        +--+-------+                    +---+------+
           |                                |
     10.0.0.0/24                      192.168.1.0/24
           |                                |
      +----+----+                     +-----+-----+
      |  PC A   |                     |  PC B     |
      |10.0.0.10|                     |192.168.1.10|
      +---------+                     +-----------+

R1 Routing Table

Destination	Next Hop	Interface	Metric	Type
10.0.0.0/24	--	eth0	0	Connected
172.16.0.0/30	--	eth1	0	Connected
192.168.1.0/24	172.16.0.2	eth1	1	Static

R2 Routing Table

Destination	Next Hop	Interface	Metric	Type
172.16.0.0/30	--	eth0	0	Connected
192.168.1.0/24	--	eth1	0	Connected
10.0.0.0/24	172.16.0.1	eth0	1	Static

Packet Flow: PC A -> PC B

1. PC A (10.0.0.10 -> 192.168.1.10)
   -> Dest not local, send to Default Gateway
   -> Sends to R1 (10.0.0.1)

2. R1 receives packet
   -> Checks routing table for 192.168.1.10
   -> Match: 192.168.1.0/24 via 172.16.0.2
   -> Forwards packet to R2 (172.16.0.2)

3. R2 receives packet
   -> Checks routing table for 192.168.1.10
   -> Match: 192.168.1.0/24 directly connected on eth1
   -> ARP for 192.168.1.10 -> forwards to PC B

Configuring Static Routes (Cisco-style)

R1 Configuration

interface eth0
 ip address 10.0.0.1 255.255.255.0
 no shutdown

interface eth1
 ip address 172.16.0.1 255.255.255.252
 no shutdown

ip route 192.168.1.0 255.255.255.0 172.16.0.2

R2 Configuration

interface eth0
 ip address 172.16.0.2 255.255.255.252
 no shutdown

interface eth1
 ip address 192.168.1.1 255.255.255.0
 no shutdown

ip route 10.0.0.0 255.255.255.0 172.16.0.1

7. Exercises

Exercise 1: Routing Table Lookup

Scenario

You are the network administrator for a small company. Router R1 has the following routing table:

R1 Routing Table:
Destination         Next Hop           Interface    Metric
0.0.0.0/0           203.0.113.1        eth2         0
10.0.0.0/8          10.0.0.1           eth0         0
10.10.0.0/16        172.16.0.2         eth1         5
10.10.10.0/24       172.16.0.3         eth1         10
172.16.0.0/16       Direct             eth1         0
203.0.113.0/24      Direct             eth2         0

Questions

Packet arrives with destination IP 10.10.10.50
- Which route matches? Why?
- What is the next hop?
- Which interface will the packet exit?
Packet arrives with destination IP 10.10.5.5
- Which route matches? Why?
Packet arrives with destination IP 8.8.8.8
- Which route matches? Why?
Packet arrives with destination IP 203.0.113.25
- Which route matches? Why?
Link to 172.16.0.3 goes down
- What happens to traffic for 10.10.10.0/24?
- Does the router have a backup path?

Click for Answers

10.10.10.50
- Matches 10.10.10.0/24 -- longest prefix match (24 bits)
- Next hop: 172.16.0.3
- Interface: eth1
- Why not 10.10.0.0/16? /24 is more specific than /16
10.10.5.5
- Matches 10.10.0.0/16 -- 16-bit prefix (10.10.5.5 is within 10.10.0.0/16)
- Does NOT match 10.10.10.0/24 (5 is not in range 0-255 of the /24)
- Next hop: 172.16.0.2 via eth1
8.8.8.8
- Matches default route 0.0.0.0/0 -- no other specific route matches
- Next hop: 203.0.113.1 via eth2
203.0.113.25
- Matches directly connected network 203.0.113.0/24
- Routes directly -- no next hop needed
- Interface: eth2
Link failure to 172.16.0.3
- Traffic for 10.10.10.0/24 would fail (route via 172.16.0.3 is down)
- The router still has 10.10.0.0/16 via 172.16.0.2, but 10.10.10.0/24 is a SUBNET of 10.10.0.0/16
- Depending on the routing implementation, traffic may fall back to the less-specific /16 route, OR packets may be dropped if the /24 route was explicitly removed.
- In most dynamic implementations, there is NO automatic fallback because the /24 is manually configured.

Exercise 2: Build a Small Network with Static Routes

Scenario

You have three routers connected as follows:

                    10.0.0.0/30
         +----------------------------------+
         |                                  |
    +----+-----+                     +------+---+
    |          |    172.16.0.0/30    |          |
    |   R1     |<------------------->|   R2     |
    |          |    .1           .2  |          |
    +----+-----+                     +------+---+
         | 192.168.1.0/24                   | 192.168.2.0/24
         |                                  |
    +----+-----+                      +-----+----+
    | PC-A     |                      | PC-B     |
    | 192.168. |                      | 192.168. |
    | 1.10/24  |                      | 2.10/24  |
    +----------+                      +----------+

Interface Details

Router	Interface	IP Address	Connected To
R1	eth0	192.168.1.1/24	PC-A / LAN
R1	eth1	10.0.0.1/30	R2 eth0
R1	eth2	172.16.0.1/30	R2 eth1
R2	eth0	10.0.0.2/30	R1 eth1
R2	eth1	172.16.0.2/30	R1 eth2
R2	eth2	192.168.2.1/24	PC-B / LAN

Tasks

Task A: Write the static route commands for R1 so that PC-A can reach:

PC-B (192.168.2.10)
The Internet (via next hop 10.0.0.2)

Task B: Write the static route commands for R2 so that PC-B can reach:

PC-A (192.168.1.10)
The Internet (via next hop 172.16.0.1)

Task C: Configure a floating static route as a backup:

On R1, add a backup route to 192.168.2.0/24 via 172.16.0.2 with AD 10
Which path does R1 normally use? Which one becomes active when the primary fails?

Task D: PC-A (192.168.1.10) pings PC-B (192.168.2.10).

Trace the packet path step by step.
What default gateway does PC-A use?
What does R1 do when it receives the packet?

Click for Answers

Task A -- R1 Static Routes

ip route 192.168.2.0 255.255.255.0 10.0.0.2        ! Primary to PC-B's network
ip route 0.0.0.0 0.0.0.0 10.0.0.2                 ! Default route to Internet

Task B -- R2 Static Routes

ip route 192.168.1.0 255.255.255.0 172.16.0.1     ! Primary to PC-A's network
ip route 0.0.0.0 0.0.0.0 172.16.0.1              ! Default route to Internet

Task C -- Floating Static Route (Backup)

ip route 192.168.2.0 255.255.255.0 172.16.0.2 10  ! AD 10 = backup

Primary path: R1 -> R2 via 10.0.0.0/30 (AD 1, the direct static route)
Backup path: R1 -> R2 via 172.16.0.0/30 (AD 10, floating static)
If the 10.0.0.0/30 link fails, the floating route with higher AD (10) activates automatically
When the primary link comes back, the backup route is removed (AD 1 wins over AD 10)

Task D -- Packet Trace (PC-A -> PC-B)

PC-A (192.168.1.10) checks: 192.168.2.10 is NOT on my network (192.168.1.0/24)
PC-A sends packet to its default gateway: 192.168.1.1 (R1's eth0)
R1 receives the packet on eth0:
- Destination: 192.168.2.10
- Looks up routing table -> matches 192.168.2.0/24 via 10.0.0.2
- Forwards packet to 10.0.0.2 (R2's eth0) via eth1
R2 receives the packet on eth0:
- Destination: 192.168.2.10
- Matches directly connected network 192.168.2.0/24 on eth2
- ARP for 192.168.2.10 -> delivers to PC-B

Routing Fundamentals: Understanding How Routers Work, Decoding the Routing Table with Longest Prefix Match, and the Role of the Default Gateway in Network Communication

Table of Contents

1. What is Routing?

Analogy: The Postal System

Routers vs. Switches

2. How Routing Works

Step-by-Step Process

The Packet Journey

Key Routing Principle

3. Routing Table

What is a Routing Table?

Anatomy of a Routing Table Entry

Field Breakdown

Types of Routes

1. Directly Connected Routes

2. Static Routes

3. Dynamic Routes

Routing Decision Logic

Longest Prefix Match Rule

4. Default Gateway

What is a Default Gateway?

Where It Lives

How a Device Uses Its Default Gateway

Checking Your Default Gateway

Windows

Linux / macOS

Example Output

The Default Route in Routing Tables

Why Default Gateway Matters

5. Key Concepts & Terminology

Common AD Values

6. Hands-On Example: Building a Simple Routing Table

Network Topology

R1 Routing Table

R2 Routing Table

Packet Flow: PC A -> PC B

Configuring Static Routes (Cisco-style)

R1 Configuration

R2 Configuration

7. Exercises

Exercise 1: Routing Table Lookup

Scenario

Questions

Exercise 2: Build a Small Network with Static Routes

Scenario

Interface Details

Tasks

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