Internet Layer

IP Header

Protocol Field in IP Header


Protocol Field in IP Header

ICMP

Internet Control Message Protocol (ICMP) works at the Network layer and is used by IP for many different services.

• ICMP is a management protocol and messaging service provider for IP.
• Its messages are carried as IP datagrams.

ICMP packets have the following characteristics:

• They can provide hosts with information about network problems.
• They are encapsulated within IP datagrams.

E0 of LAB_B goes down. What happens?


ARP
ARP resolves IP addresses to Ethernet (MAC) addresses.


RARP

Host to Host Layer

The main purpose of the Host-to-Host layer is to shield the upper-layer applications from the complexities of the network.

This layer says to the upper layer, “Just give me your data stream, with any instructions, and I’ll begin the process of getting your information ready to send.”

The following sections describe the two protocols at this layer:

• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)

TCP
The figure shows the different fields within the TCP header.


UDP
This figure clearly illustrates UDP’s markedly low overhead as compared to TCP’s hungry usage.


Key concepts of Host to Host Protocols

TCP:

• Sequenced
• Reliable
• Connection-oriented
• Virtual circuit
• Acknowledgments
• Windowing flow control

UDP:

• Unsequenced
• Unreliable
• Connectionless
• Low overhead
• No acknowledgment
• No windowing or flow control

Port Numbers
Port number examples for TCP and UDP


Key Protocols and Port Numbers

TCP:
Telnet = 23
SMTP = 25
HTTP = 80
FTP = 21
DNS = 53
HTTPS = 443


UDP:
SNMP = 161
TFTP = 69
DNS = 53

TCP/IP and the DoD Model

The figure shows a comparison of the DoD model and the OSI reference model. As you can see, the two are similar in concept, but each has a different number of layers with different names.


The TCP/IP Protocol Suite


The DoD and OSI models are alike in design and concept and have similar functions in similar layers.


Process/Application Layer

This section describes different applications and services typically used in IP networks. The following protocols and applications are discussed:

• Telnet
• FTP
• TFTP
• NFS
• SMTP
• LPD
• X Window
• SNMP
• DNS
• DHCP/BootP

Ethernet Cabling

Ethernet cabling is an important discussion, especially if you are planning on taking the Cisco exams.

Three types of Ethernet cables are available:

• Straight-through cable
• Crossover cable
• Rolled cable

Straight Through

The straight-through cable is used to connect

• Host to switch or hub
• Router to switch or hub

Crossover Cable

The crossover cable can be used to connect

• Switch to switch
• Hub to hub
• Host to host
• Hub to switch
• Router direct to host

Rolled Cable

Although rolled cable isn’t used to connect any Ethernet connections together, you can use a rolled Ethernet cable to connect a host to a router console serial communication (com) port.


Data Encapsulation

When a host transmits data across a network to another device, the data goes through encapsulation:

• It is wrapped with protocol information at each layer of the OSI model.
• Each layer communicates only with its peer layer on the receiving device.

PDU


Port Numbers


The Transport layer uses port numbers to define both the virtual circuit and the upper-layer process.


Cisco’s Three-Layer Model


The following are the three layers and their typical functions:

• The core layer: backbone
• The distribution layer: routing
• The access layer: switching