HeaderStack CCNA
RFC-style header diagrams for IPv4, IPv6, TCP, UDP, application protocols, and wireless helpers. Click any field box to expand details.
L2 = FrameL3 = PacketL4 = Segment / DatagramL7 = PayloadTrailer
CCNA priority moduleMost tested L3 topic
Layer 3: IPv4
20-byte base header (RFC 791) — one of the most tested CCNA topics
IPv4 header layout
32-bit words · scroll on small screens →bits 0 – 15
bits 16 – 31
1
2
3
4
5
6
Payload
TCP segment, UDP datagram, or ICMP message follows the IP header
Row numbers match 32-bit words. Tap any field to see the full description below.
Protocol field values
ICMP = 1
TCP = 6
UDP = 17
ICMPv6 = 58
All fields
Tap a field to highlight it in the diagram above
Visual study notes
- →TTL decreases by 1 at every router.
- →Header checksum changes when TTL changes.
- →Source and destination IP usually remain the same end-to-end.
- →Protocol field tells Layer 3 what Layer 4 protocol is inside: TCP = 6, UDP = 17, ICMP = 1.
General L2/L3 study notes
- • Switches forward based on destination MAC addresses within a broadcast domain.
- • Routers remove the old Layer 2 frame and build a new one for the next hop.
- • Layer 3 source and destination IP addresses usually stay the same across routed hops.
- • Layer 2 source and destination MAC addresses change at every router hop.
- • ARP resolves IP → MAC on the local segment before an IPv4 packet can be encapsulated in Ethernet.