Layer 3 IP Models
How IP is modeled
Packet Tracer 4.11 models only certain aspects of IP.
- IP version 4 is modeled.
- IP addresses are 32 bits long, and displayed using the dot-decimal
notation, where each octet or byte of the IP address is displayed in decimal
number and separated by a dot.
- All class A, B, and C addresses (1.0.0.0 to 224.255.255.255) except
loopback addresses (127.0.0.0 to 127.255.255.255) can be assigned to ports.
- All IP packet fields are displayed in the PDU details. However, only
the following four fields are in used: Destination IP Address, Source IP
Address, TTL, and Protocol.
- The Protocol field in the IP packet identifies the layer 4 PDU. When
a device sends an IP packet, it places the value corresponding to the layer 4
process or service in the Protocol field. When a device receives an IP
packet, it dispatches the layer 4 PDU to a process or service corresponding to
the value of the Protocol field.
How devices process incoming ICMP packets
When a device receives an ICMP packet:
- It checks the ICMP message contained in the packet.
- If the packet contains the message "TTL Exceeded" or "Echo Reply."
- It checks to see if it has recently sent an ICMP message with the same identification as the received ICMP message.
- If so, it sends out the ICMP.