Introduction to the TCPIP Suite

Network Devices

The main devices that interconnect networks are hubs, switches, and routers, as described in the following sections.

Note

Many other devices can be used in networks to provide specific functionality; these devices are introduced in the appropriate chapters in this book. For example, security devices, including firewalls, are discussed in Chapter 4, "Network Security Design."

Terminology: Domains, Bandwidth, Broadcast, and Multicast

The following is some terminology related to the operation of network devices:

• A domain is a specific part of a network.

• Bandwidth is the amount of data that can be carried across a network in a given time period.

• A broadcast is data meant for all devices; it uses a special broadcast address to indicate this.

• A multicast is data destined for a specific group; again, a special address indicates this.

• A bandwidth domain, known as a collision domain for Ethernet LANs, includes all devices that share the same bandwidth.

• A broadcast domain includes all devices that receive each other's broadcasts (and multicasts).

Devices in the same bandwidth domain are also in the same broadcast domain; however, devices in the same broadcast domain can be in different bandwidth domains.

Hubs

A typical Ethernet LAN uses unshielded twisted-pair (UTP) cables with RJ-45 connectors (which are slightly bigger than telephone RJ-11 connectors). Because these cables have only two ends, you need an intermediary device to connect more than two computers. That device is a hub.

A hub works at Layer 1 and connects multiple devices so that they are logically all on one LAN.

Physical Interfaces and Ports

The physical connection point on a network devicea hub, switch, or routeris called an interface or a port.

Don't confuse this definition of port with the application layer port numbers that are discussed in the "TCP/IP Transport Layer Protocols" section, later in this appendix.

A hub has no intelligenceit sends all data received on any port to all the other ports. Thus, devices connected through a hub receive everything that the other devices send, whether it was meant for them or not. This is analogous to being in a room with lots of peopleif you speak, everyone can hear you. If more than one person speaks at a time, everyone just hears noise.

All devices connected to a hub are in one collision domain and one broadcast domain.

Note

A hub just repeats all the data received on any port to all the other ports; thus, hubs are also known as repeaters.

Switches

Just as having many people in a room trying to speak can result in nobody hearing anything intelligible, using hubs in anything but a small network is not efficient. To improve performance, LANs are usually divided into multiple smaller LANs interconnected by a Layer 2 LAN switch. The devices connected to a switch again appear as they are all on one LAN, but this time, multiple conversations between devices connected through the switch can be happening simultaneously.

Note

In this appendix, we discuss Layer 2 LAN switches. Chapter 2, "Switching Design," has more advanced switching topics, including virtual LANs (VLANs) and Layer 3 switching.

Switches are Layer 2 devices and have some intelligencethey only send data to a port if the data needs to go there. A device connected to a switch port does not receive any of the information addressed to devices on other ports. Thus, the main advantage of using a switch instead of a hub is that the traffic received by a device is reduced, because only frames addressed to a specific device are forwarded to the port on which that device is connected.

Switches keep track of who is where, and who is talking to whom, and only send data where it needs to go. If the switch receives a broadcast (information meant for everyone) though, by default it sends it out all ports (except for the one on which it was received).

All devices connected to one switch port are in the same collision domain, but devices connected to different ports are in different collision domains. By default, all devices connected to a switch are in the same broadcast domain.

Switches Versus Bridges

You might have also heard of bridges. Switches and bridges are logically equivalent. The main differences are as follows:

• Switches are significantly faster because they switch in hardware, whereas bridges switch in software.

• Switches can interconnect LANs of unlike bandwidth. A 10-Mbps Ethernet LAN and a 100-Mbps Ethernet LAN, for example, can be connected using a switch. In contrast, all the ports on a bridge support one type of media.

• Switches typically have more ports than bridges.

Switches do not allow devices on different logical LANs to communicate with each other; this requires a router, as described in the next section.

Routers

A router goes one step further than a switch. It is a Layer 3 device that has a lot more intelligence than a hub or switch. By using logical Layer 3 addresses, routers allow devices on different LANs to communicate with each other and with distant devices, for example, those connected through the Internet or through a WAN.

A device connected to a router does not receive any of the information meant just for devices on other ports, or broadcasts (destined for all networks) from devices on other ports.

The router keeps track of who is where, and who is talking to whom, and only sends data where it needs to go. It supports communication between LANs, but it blocks broadcasts (destined for all networks).

All devices that are connected to one router port are in the same collision domain, but devices connected to different ports are in different collision domains.

All the devices connected to one router port are in the same broadcast domain, but devices connected to different ports are in different broadcast domains. Routers block broadcasts (destined for all networks) and multicasts by default; routers only forward unicast packets (destined for a specific device) and packets of a special type, called directed broadcasts.

Note

IP multicast technology, which enables multicast packets to be sent throughout a network, is described in Chapter 10, "Other Enabling Technologies."

Note

An IP directed broadcast is an IP packet that is destined for all devices on an IP subnet, but which originates from a device on another subnet. IP subnets are described in the "Addressing" section, later in this appendix. On Cisco routers, the ip directed-broadcast interface command controls what the router connected to the destination subnet does with a directed broadcast packet; the behavior of this command is described in Chapter 2.

The fact that a router does not forward broadcasts (destined for all networks) is a significant difference between a router and a switch, and helps to control the amount of traffic on the network. For example, many protocols, such as IP, use broadcasts for routing protocol advertisements, discovering servers, and so on. These broadcasts are a necessary part of local LAN traffic, but they are not required on other LANs and can even overwhelm slower WANs. Routers can generate broadcasts themselves if necessary (for example, to send out a routing protocol advertisement) but do not pass on a received broadcast.

Routing operation is discussed further in the "Routing" section, later in this appendix.

Note

The concepts of unicast, multicast, and broadcast apply to Layer 2 and Layer 3 separately. While a router does not forward any type of frame, it can forward a unicast, multicast, or directed broadcast packet (that it received in a frame). A switch, however, can forward a unicast, multicast, or broadcast frame.