Introduction to IP Routing

CompTIA Network+ · N10-009 · Objective 2.1

Chapter 9 — Introduction to IP Routing

Packets keep their destination; frames change their clothes. This study map follows routing-table entries, route selection, ARP, TTL, static and dynamic routes, convergence, and protocol families one hop at a time.

15 sectionsRoute selectorPacket-hop animatorAD explorerProtocol classifier15-question exam check

Backbone / Contents

01

Routing Foundations

IP routing forwards a Layer 3 packet from its source network toward a destination network. Each router uses the destination IP and its own routing table.

Exchanges routes

Routing protocol

OSPF, EIGRP, BGP, and RIP exchange reachability information and calculate paths.

Carries user packets

Routed protocol

IPv4 and IPv6 carry application data through the routed network.

Forwarding map

Routing table

Contains prefixes, next hops, interfaces, sources, administrative distances, and metrics.

Exam trapOSPF and BGP exchange routing information. IPv4 and IPv6 are the routed protocols carrying user packets.

02

How Routes Enter the Table

Automatic

Connected

Appears when an interface is up/up with an assigned prefix; disappears when the link fails.

Router itself

Local

A host route for the address assigned directly to a router interface.

Manual

Static

An administrator configures the destination and next hop or exit interface.

Catch-all

Default

0.0.0.0/0 is used when no more-specific route matches.

Protocol learned

Dynamic

A routing protocol advertises and calculates reachability, adapting after convergence.

03

Routing-Table Anatomy

Select a field to see the question it answers.

04

Route-Selection Order

1 · PREFIXLongest match

→

2 · SOURCELowest AD

→

3 · PATHBest metric

→

4 · EQUALPossible load balance

Memory aidSpecificity first, source trust second, protocol cost third: prefix length → administrative distance → metric.

05

Longest-Prefix Route Selector

Enter a destination and let the router choose the most-specific matching prefix.

Candidate	Matches?	Result

06

Administrative Distance Explorer

ScopeAD is locally significant and compares route sources for the same destination prefix. It is not an end-to-end universal path score.

07

Metrics by Protocol

Protocol	Main path measure	Association
RIP	Hop count	Distance-vector
OSPF	Cost, commonly bandwidth-derived	Link-state
EIGRP	Composite: mainly bandwidth and delay	Advanced distance-vector
BGP	Path attributes and policy	Path-vector

08

Local or Remote?

Same subnet

Direct delivery

Packet destination IP is the host; first-frame destination MAC is that same host’s MAC.

Different subnet

Gateway delivery

Packet destination IP remains the remote host; first-frame destination MAC is the default gateway interface.

Critical distinctionFor a remote destination, the first Ethernet frame targets the default gateway’s MAC—not the remote host or switch management MAC.

09

Packet-Hop Animator

Trace packet creation, routing, ARP, frame removal, TTL change, and re-encapsulation.

Host A creates application data.

10

What Changes at Each Hop?

Field/object	End-to-end	Per-hop behavior
Source IP	Normally remains Host A	Not replaced by router
Destination IP	Normally remains Host B	Used for each route lookup
Source MAC	Changes	Sender on current link
Destination MAC	Changes	Next hop on current link
Ethernet FCS	Recalculated	New frame, new check
TTL	Decreases	Reduced by each router

Core ruleThe IP endpoints normally stay constant. Each router removes the old Layer 2 frame and builds a new frame for the next local link.

11

ARP, Failures & Return Path

Return pathOutbound success is only half the conversation. Every reply needs a valid permitted route back to the original source network.

12

Static versus Dynamic Routing

Manual control

Static routing

Low overhead and predictable, but maintenance and failure response are manual unless tracking is configured.

Adaptive

Dynamic routing

Routers exchange and calculate routes, consume control resources, and reconverge after changes.

Backup

Floating static

Uses deliberately higher AD so it activates only after the preferred dynamic route disappears.

Stable knowledge

Convergence

Routers detect change, exchange updates, recalculate, and install consistent routes.

13

Dynamic Routing Families

Autonomous systemAn AS is a collection of networks under common administrative control and routing policy. IGPs operate inside; BGP exchanges policy and reachability between ASes.

14

PowerShell 7 Practice

Get-NetIPConfigurationAddresses, gateways, interface state

Get-NetRoute -AddressFamily IPv4IPv4 routing table

Get-NetRoute -DestinationPrefix 0.0.0.0/0Default route

Get-NetNeighbor -AddressFamily IPv4ARP mappings

Test-NetConnection 1.1.1.1Remote reachability and interface

Test-NetConnection example.com -TraceRouteRouted path

tracert -d 1.1.1.1Trace without DNS delays

route print -4Classic IPv4 route table

15

Exam Check

Fifteen review questions drawn directly from the Chapter 9 manual.

Score: 0 / 0

NETWORK+ STUDY MAP · CHAPTER 9 · INTRODUCTION TO IP ROUTING