Variable Length Subnet Masks (VLSM)

Subnets with no VLSM applied



Subnets with VLSM applied



VSLM, Example 1



VLSM, Example 2


Summary Example

Subnetting Basics

Benefits of subnetting include:

• Reduced network traffic
• Optimized network performance
• Simplified management
• Facilitated spanning of large geographical distances.

How To Create Subnets

Take bits from the host portion of the IP address and reserve the to divine the subnet address.


Understanding the Powers of 2


Subnet Masks

• Used to define which part of the host address will be used as the subnet address.
• A 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion.

Default Subnet Masks


Classless Inter-Domain Routing (CIDR)

Used to allocate an amount of IP address space to a given entity (company, home, customer, etc).
Example: 192.168.10.32/28

The slash notation (/) means how many bits are turned on (1s) and tells you what your subnet mask is.

CIDR Values

IP Addressing

An IP address is a numeric identifier assigned to each machine on an IP network.

It designates the specific location of a device on the network.

IP addressing was designed to allow hosts on one network to communicate with a host on a different network regardless of the type of LANs the hosts are participating in.


IP Terminology

BIT: A bit is one digit, either a 1 or a 0.

BYTE: A byte is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits.

OCTET: An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms byte and octet are completely interchangeable.

Network address: This is the designation used in routing to send packets to a remote network—for example, 10.0.0.0, 172.16.0.0, and 192.168.10.0.

Broadcast address: The address used by applications and hosts to send information to all nodes on a network is called the broadcast address.


Network Addressing


Subdividing an IP address into a network and node address is determined by the class designation of one’s network. This figure summarizes the three classes of networks.


Reserved Addressing


Private Addressing

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

The OSI Model

A hub in an network


A Switch in an network


Ethernet Networking

• Ethernet is a contention media access method that allows all hosts on a network to share the same bandwidth of a link.
• Ethernet is popular because it’s readily scalable, meaning that it’s comparatively easy to integrate new technologies, such as Fast Ethernet and Gigabit Ethernet, into an existing network infrastructure.
• It’s also relatively simple to implement in the first place, and with it, troubleshooting is reasonably straightforward.

Ethernet Collision Detection
CSMA/CD


Half and Full Duplex

Half-duplex Ethernet is defined in the original 802.3 Ethernet; Cisco says it uses only one wire pair with a digital signal running in both directions on the wire.

But full-duplex Ethernet uses two pairs of wires instead of one wire pair like half duplex. And full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device.

Full-duplex Ethernet can be used in three situations:

• With a connection from a switch to a host
• With a connection from a switch to a switch
• With a connection from a host to a host using a crossover cable

Internetworking Basics

How would you say the PC named Bob communicates with the PC named Sally?


Switches can replace the hub, breaking up collision domains.



Keep in mind that the hub used in the figure just extended the one collision domain from the switch port.


Here’s a list of some of the things that commonly cause LAN traffic congestion:

• Too many hosts in a broadcast domain
• Broadcast storms
• Multicasting
• Low bandwidth
• Adding hubs for connectivity to the network
• A bunch of ARP or IPX traffic (IPX is a Novell protocol that is like IP, but really, really chatty. Typically not used in today’s networks.)

Routers create an internetwork.


There are two advantages of using routers in your network:

• They don’t forward broadcasts by default.
• They can filter the network based on layer 3 (Network layer) information (e.g., IP address).

Four router functions in your network can be listed as follows:

• Packet switching
• Packet filtering
• Internetwork communication
• Path selection

Internetworking devices


Switched networks creating an internetwork

The Layered Approach

• A reference model is a conceptual blueprint of how communications should take place.
• It addresses all the processes required for effective communication and divides these processes into logical groupings called layers.
• When a communication system is designed in this manner, it’s known as layered architecture.

The OSI Model

The OSI isn’t a physical model. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network.

It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.


The upper layers



The lower layers



The Layer Functions



Connection-Oriented Communication



Windowing


Network Layer Routing Table used in a router



Router in an internetwork



Data Link Layer