Networks come in a wide variety of types. The most common are LANs and WANs but there are many other types of networks, including metropolitan area networks (MANs) storage area networks (SANs) content networks (CNs) intranets and extranets VPNs and others.

Local Area Networks

Local area networks (LANs) are used to connect networking devices that are in a very close geographic area, such as a floor of a building, a building itself or a campus environment. In a LAN you’ll find PCs file servers hubs bridges switches routers multilayer switches voice gateways, firewalls, and other devices.

The media types used in LANs include Ethernet, Fast Ethernet (FE), Gigabit Ethernet (GE), Token Ring, and FDDI. Today, most networks use some form of Ethernet.

Wide Area Networks

Wide area networks (WANs) are used to connect LANs together. Typically, WANs are used when the LANs that must be connected are separated by a large distance. Whereas a corporation provides its own infrastructure for a LAN, WANs are leased from carrier networks, such as telephone companies.

Four basic types of connections, or circuits, are used in WAN services: circuit-switched, cell-switched, packet-switched, and dedicated connections.

Cisco has developed a three-layer hierarchical model to help you design campus networks. Cisco uses this model to simplify designing, implementing, and managing large-scale networks. With traditional network designs, it was common practice to place the networking services at the center of the network and the users at the periphery.

However, many things in networking have changed over the past decade, including advancements in applications, developments in graphical user interfaces (GUIs), the proliferation of multimedia applications, the explosion of the Internet, and fast-paced changes in your users traffic patterns. Cisco developed the three-layer model to accommodate these rapid changes.

Cisco’s hierarchical model contains three layers: core, distribution, and access. A well-designed network typically follows this topology.

Core Layer
The core layer, as its name suggests, is the backbone of the network. It provides a high speed connection between the different distribution layer devices. Because of the need for high-speed connections, the core consists of high-speed switches and will not, typically, perform any type of packet or frame manipulations, such as filtering or Quality of Service.

Because switches are used at the core, the core is referred to as a layer-2 core. The traffic that traverses the core is typically to access enterprise corporate resources: connections to the Internet, gateways, e-mail servers, and corporate applications.

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. Certainly, the IEEE specifications discuss the process of half duplex somewhat differently, but what Cisco is talking about is a general sense of what is happening here with Ethernet.

It also uses the CSMA/CD protocol to help prevent collisions and to permit retransmitting if a collision does occur. If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions. Half-duplex Ethernet typically 10BaseT is only about 30 to 40 percent efficient as Cisco sees it, because a large 10BaseT network will usually only give you 3 to 4Mbps at most.

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. This means that with full-duplex data transfer, you get a faster data transfer compared to half duplex. And because the transmitted data is sent on a different set of wires than the received data, no collisions will occur.
The reason you don’t need to worry about collisions is because now it’s like a freeway with multiple lanes instead of the single-lane road provided by half duplex.

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 uses both Data Link and Physical layer specifications, and this section of the chapter will give you both the Data Link and Physical layer information you need to effectively implement, troubleshoot, and maintain an Ethernet network.

Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/ CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes. And trust me good collision management is crucial, because when a node transmits in a CSMA/CD network, all the other nodes on the network receive and examine that transmission. Only bridges and routers can effectively prevent a transmission from propagating throughout the entire network!

The types of Ethernet cables available are:
- Straight-through cable
- Crossover cable
- Rolled cable

Straight-Through Cable

The straight-through cable is used to connect
- Host to switch or hub
- Router to switch or hub

Four wires are used in straight-through cable to connect Ethernet devices. It is relatively simple to create this type; Figure as below shows the four wires used in a straight-through Ethernet cable.

Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6, and you’ll be up and networking in no time. However, remember that this would be an Ethernet-only cable and wouldn’t work with Voice, Token Ring, ISDN, etc.

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

The same four wires are used in this cable as in the straight-through cable; we just connect different pins together. Figure as below shows how the four wires are used in a crossover Ethernet cable.
Notice that instead of connecting 1 to 1, etc., here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

Ethernet at the Physical Layer
Ethernet was first implemented by a group called DIX (Digital, Intel, and Xerox). They created and implemented the first Ethernet LAN specification, which the IEEE used to create the IEEE 802.3 Committee. This was a 10Mbps network that ran on coax, and then eventually twistedpair and fiber physical media.

The IEEE extended the 802.3 Committee to two new committees known as 802.3u (Fast Ethernet) and 802.3ab (Gigabit Ethernet on category 5) and then finally 802.3ae (10Gbps over fiber and coax).

Figure as below shows the IEEE 802.3 and original Ethernet Physical layer specifications. When designing your LAN, it’s really important to understand the different types of Ethernet media available to you. Sure, it would be great to run Gigabit Ethernet to each desktop and 10Gbps between switches, and although this might happen one day, justifying the cost of that network today would be pretty difficult. But if you mix and match the different types of Ethernet media methods currently available, you can come up with a cost-effective network solution that works great.

Ethernet at the Data Link Layer
Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to as hardware addressing or MAC addressing. Ethernet is also responsible for framing packets received from the Network layer and preparing them for transmission on the local network through the Ethernet contention media access method. There are four different types of Ethernet frames available:

- Ethernet_II
- IEEE 802.3
- IEEE 802.2
- SNAP

Ethernet Addressing

Here’s where we get into how Ethernet addressing works. It uses the Media Access Control (MAC) address burned into each and every Ethernet Network Interface Card (NIC). The MAC, or hardware address, is a 48-bit (6-byte) address written in a hexadecimal format. Figure as below shows the 48-bit MAC addresses and how the bits are divided.

You known about Cisco’s three-layer hierarchical model for network design: core layer, distribution layer, and access layer. Once you have designed your network and have decided on the types of devices you’ll be using at each of the three layers, you must then pick a specific product for each of these devices. When choosing a networking product, consider the following:

- Is the product easy to install and support?
- Does the product provide the necessary features/functions to meet your networking requirements?
- Does the product support enough ports and offer enough backplane capacity to meet your network’s growth and bandwidth requirements?
- Is the product reliable, and can it provide redundancy?
- If it is a layer-3 device, does the product provide support for both mobile users and branch office connections?
- Can the product be easily upgraded, protecting your investment in the product?

If you are implementing a WAN solution, you should consider the following when making a choice:

- Make sure the solution is cost-effective.
- Make sure the service you want to use is available in the location where you will be installing it. Some services, such as ATM, DSL, and ISDN, are not available in all areas.
- Make sure the solution you choose provides the necessary amount of bandwidth for your user’s needs.

Using the ping Command
To see all the different protocols that you can use with the ping program, type ping ? :

Router#ping ?
WORD Ping destination address or hostname
apollo Apollo echo
appletalk Appletalk echo
clns CLNS echo
decnet DECnet echo
ip IP echo
ipx Novell/IPX echo
srb srb echo
tag Tag encapsulated IP echo
vines Vines echo
xns XNS echo
<cr>

The ping output displays the minimum, average, and maximum times it takes for a ping packet to find a specified system and return. Here’s an example:

Router#ping RouterA
Translating “RouterA”…domain server (192.168.0.1)[OK]
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/32 ms
Router#

You can see that the DNS server was used to resolve the name, and the device was pinged in 32ms (milliseconds).

Using the traceroute Command
Traceroute (the traceroute command, or trace for short) shows the path a packet takes to get to a remote device. To see the protocols that you can use with the traceroute command, type traceroute ? :