CCNA Networking · Quick Reference

Module 8 — Network Layer Cheat Sheet

CCNA Networking

In one line: Layer 3 — the Network Layer — provides end-to-end logical addressing and routing across multiple networks. Routers operate at Layer 3, reading IP headers to forward packets towa...

Key Ideas

1Network Layer Characteristics. Layer 3 — the Network Layer — provides end-to-end logical addressing and routing across multiple networks. Routers operate at Layer 3, reading IP headers to forward pa...

2IPv4 Packet Header. Every IPv4 packet has a header of at least 20 bytes (5 × 32-bit words). Knowing each field and its purpose is a CCNA exam requirement. The header is followed by the up...

3IPv6 Packet Header. IPv4 addresses were exhausted: IANA ran out in 2011, APNIC (Asia-Pacific) in 2011, and ARIN (North America) in 2015. IPv6 solves this with 128-bit addresses — enough f...

4How Hosts Make Forwarding Decisions. Every time a PC wants to send a packet, it must answer one question: is the destination on my local network, or on a remote network?

5Host Routing Tables. Every device — including PCs and laptops — maintains a routing table that determines where to send packets based on the destination IP address. A typical PC routing ta...

Code Examples

Internet Protocol Version 4
  Version: 4
  Header Length: 20 bytes (5)
  Differentiated Services: 0x00 (DSCP: Default, ECN: Not-ECT)
  Total Length: 84
  Identification: 0x1234 (4660)
  Flags: 0x40 (Don't Fragment)
  Fragment Offset: 0
  Time to Liv...

! PC1: IP 192.168.1.10/24, Gateway: 192.168.1.1
! Wants to reach 10.0.0.5

! Is 10.0.0.5 on same network?
! 192.168.1.10 AND 255.255.255.0 = 192.168.1.0  (my network)
! 10.0.0.5    AND 255.255.255.0 = 10.0.0.0     (different!)
! → Different network → ...

! Windows
C:\> route print
Network Destination  Netmask          Gateway        Interface
0.0.0.0              0.0.0.0          192.168.1.1    192.168.1.10  ! Default route
127.0.0.0            255.0.0.0        On-link        127.0.0.1      ! Loopbac...