Nano Ink

Routing

Routing

Routing is the process of selecting the best path for data packets to travel from a source device to a destination device across a network.

Key Components

  • Routers - Devices that forward packets between networks
  • Routing Tables - Store information about network paths
  • Routing Protocols - Rules for determining best paths

Types of Routing

Type Description
Static Routing Manually configured paths
Dynamic Routing Automatic path selection

Routing Protocols Overview

Routing protocols decide HOW routers choose paths.

Two Main Categories

Type Example Used For
Interior Gateway Protocol (IGP) OSPF Inside one network
Exterior Gateway Protocol (EGP) BGP Between networks

OSPF – Open Shortest Path First

  • Used INSIDE one organization or ISP
  • Calculates shortest path using Dijkstra algorithm
  • Based on cost, bandwidth, and speed
  • Fast and dynamic

Used In

  • Company networks
  • University networks
  • Internal ISP routing

Goal

Find the best path inside a single network.

BGP – Border Gateway Protocol

  • Used BETWEEN different networks and ISPs
  • Runs the entire internet
  • Policy-based routing
  • Does NOT care about speed or distance

What BGP Cares About

  • Business agreements
  • Policies
  • Stability
  • AS-PATH

Difference Between OSPF and BGP

Feature OSPF BGP
Type Interior (IGP) Exterior (EGP)
Used In Inside a network Between networks
Goal Shortest path Best policy path
Algorithm Dijkstra Path Vector
Internet Usage No Yes

Protocol Comparison

Protocol Cares About
OSPF Speed / cost / shortest path
RIP Hop count (distance)
BGP Policies and preferences

Autonomous System (AS)

Definition

An Autonomous System is a large network under single administrative control.

AS Number (ASN)

  • Unique identifier for each network on the internet
  • Examples:
    • Google: AS15169
    • Facebook: AS32934

AS-PATH in BGP

  • AS-PATH is a list of AS numbers a packet passes through
  • Used for:
    • Loop prevention
    • Path selection

Example AS-PATH

AS100 → AS200 → AS32934

What Happens When You Ping a Website

Example: ping facebook.com

Steps:

  1. DNS Resolution

    • Domain name is converted to IP address
    • Example: facebook.com → 57.144.56.1

  2. ICMP Packet Creation

    • System creates an ICMP Echo Request packet

  3. Routing Table Lookup

    • OS checks where to send packet next

  4. ARP Resolution

    • Finds MAC address of the gateway

  5. Packet Sent to Router

  6. Routers Forward Packet Across Internet

  7. Facebook Server Responds

  8. Reply Returns to Your Device

Traceroute

Traceroute shows the path packets take across the internet.

Command

traceroute facebook.com

What Traceroute Shows

  • Each router hop
  • Latency between hops
  • Network path structure

Important Notes

  • Some hops show * * * due to ICMP blocking
  • Final hops may be hidden for security

Packets

Data in network travels in small chunks called packets.

A packet contains:

  • Source IP
  • Destination IP
  • Data payload

Important Commands

View Routing Table

ip route

Trace Route

traceroute facebook.com

View ARP Table

ip neigh

Quick Reference

Concept Description
Routing Selecting best path for packets
Router Device that forwards packets
OSPF Interior routing protocol (shortest path)
BGP Exterior routing protocol (policy-based)
AS Network under single admin control
Traceroute Shows path packets take
Static Routing Manually configured
Dynamic Routing Automatically determined

Summary

  • OSPF is used inside networks
  • BGP is used between networks
  • Traceroute shows path, not protocol
  • Internet routing is mostly BGP
  • Your PC only knows next hop, not full path