CCNA ITN: Module 8

network layer

OSI Layer 3, enables end devices to exchange data across networks

network protocols

functions include addressing, encapsulation, routing, de-encapsulation

IP protocol

connectionless, best effort, media independent

connectionless

IP sends data without setting up a session

IP header

no need for session tracking fields, reducing overhead

best effort

IP doesn’t guarantee delivery or reliability

upper layers

handle packet loss, errors, or ordering (e.g., TCP)

media independent

IP works over any physical medium, limited by MTU

IPv4 header

fields include version, DiffServ, TTL, protocol, checksum, source and destination IPs

TTL

field decrements by one per hop, discarded at zero

protocol

field indicates upper-layer protocol (TCP, UDP, etc.)

IPv4 source/destination addresses

identify sending and receiving devices

fragmentation

fields

IPv4 limitations

address depletion, NAT complexity, lack of end-to-end connectivity

IPv6

128-bit addresses, simpler header, improved performance, no NAT needed

IPv6 header

fields: version, traffic class, flow label, payload length, next header, hop limit

IPv6 addressing

source and destination fields are 128-bit each

IPv6 extension headers

provide optional features like fragmentation, security, mobility

routing

table used to determine best path for forwarding packets

host routing table

contains entries for loopback, local, and remote hosts

loopback addresses

127.0.0.1 for IPv4 and ::1 for IPv6

default gateway

router interface on local network that routes packets to other networks

default gateway in IPv4

obtained manually or via DHCP

default gateway in IPv6

obtained from router advertisement or manual config

default route

entry used when no specific route to destination exists

Windows route table

viewed using route print or netstat -r

routing table lists

interface MACs, IP routes, and gateway info

router examines

destination IP to decide how to forward a packet

routing table entries

directly connected, remote, and default routes

directly-connected route

network on an active interface

remote network route

connected through other routers

default route

used when no specific match is found

static route

manually configured, not automatically updated

static route best for

small/simple networks with stable topology

dynamic routing

uses protocols like OSPF, EIGRP to share routes

dynamic routing

auto-discovers networks and adapts to changes

show ip route

displays IPv4 routing table with coded entries

routing codes

L (local), C (connected), S (static), O (OSPF), E (EIGRP)