- Internetworking basics which discussed the meaning of the term and provided a basic example of two hosts communicating with each other.
- Network segmentation which covered the different types of network devices (hubs, switches, bridges and routers); and how they can be employed to physically segment a network.
The chapter began with a brief look at the exponential growth of networks and the networking field over the last 15 years or so. Essentially today's networks have evolved in order to keep up with the substantial increases in basic mission-critical user needs, including data sharing and networked printers as well as more complex requirements such as videoconferencing.
The term internetwork was introduced to describe the connection of two or more LANs or WANs via a router. These networks must be configured using a logical network addressing scheme. An example containing two computers is then depicted to show several networking concepts working in unison. One of these concepts is the use of Media Access Control (MAC) addresses, also known as hardware addresses, which are essentially "burned" into the host's Network Interface Controller (NIC) when it is manufactured. The Address Resolution Protocol (ARP) is used to translate IP addresses into MAC addresses and receives a brief mention. ARP is a fairly simple protocol that relies on the use of broadcast addresses (MAC broadcast is "ff:ff:ff:ff:ff:ff") to broadcast ARP requests in the form:
Who has 192.168.0.25? Tell 192.168.0.4
The host configured with that IP address will receive the broadcast and send an ARP request directly to the requestee's IP address with a message such as:
192.168.0.25 is at 00:0b:db:94:d2:e4
The book delves a bit deeper and talks about name resolution to translate names to their corresponding IP addresses, describing domain name resolution using the Domain Name System (DNS) and also, locally, NetBIOS name resolution using NetBIOS Name Service (NBNS) in Microsoft Windows LANs. The later is fairly similar to ARP in that it uses the broadcasting method to translate a hostname such as "alpha_XP" to its corresponding IP address, for example "192.168.0.42".
Four types of network devices are outlined. These are hubs, switches, bridges and routers. Switches and routers are really the only ones still in use today, however it is still important to know how hubs and bridges actually work so that we can understand the distinctions between all of the different devices. This is where the concept of network segmentation comes in. There are two kinds of segmentation:
- Collision domains: A collision domain is a term that's used to describe a situation where it is possible for frames sent by two or more hosts at the same time to collide with one another which then neccesitates retransmission after a certain backoff period. It is important to break up collision domains in order to improve performance
- Broadcast domains: A broadcast domain is a term that's used to describe a network segment where broadcast traffic reaches all devices in the segment.
The four network devices described are:
- Hubs (also known as multiport repeaters): Hubs just forward the frames that they receive out of all ports except for the port from which the frame originated. This means that all ports operate within a single collision and broadcast domain. This can sometimes cause congestion as the number of network hosts increases.
- Switches (also known as multiport bridges): Switches forward and filter packets from one port to another within a network, however they cannot forward or filter packets between different networks like routers can. Switches break up collision domains as each and every port on a switch represents its own collision domain but they do not segment the broadcast domain (broadcast traffic is passed through all ports).
- Bridges: Bridges work just like switches, however they generally only consist of a few ports, whereas switches often consist of dozens of ports. Bridges have been mostly replaced by switches which provide more ports and some additional functionality (i.e. greatly enhanced management ability). Like switches, bridges segment each port into its own collision domain but they do not segment the broadcast domain (broadcast traffic is passed through all ports).
- Routers (also known as layer 3 switches): Routers interconnect different networks and enable internetwork communication by providing a number of routing functions including packet switching, filtering, and path selection. They use a routing table in oder to make routing decisions and forward packets to the correct remote network. Routers can also provide connections to WAN services through serial interfaces (v.35 physical interfaces). Routers break up collision domains as well as broadcast domain by default, that is, they provide a separate broadcast and collision domain for each interface.
As you can see, switches and bridges can be used to segment networks but they do not isolate broadcast traffic.
Why is is the ability of routers to break up broadcast domains so important?
When a host sends a network broadcast, every device in that broadcast domain receives the broadcast packet and processes it to determine what to do with it (i.e. formulate an ARP reply if it has the IP address in the ARP query). The larger the broadcast domain, the more broadcast traffic overhead you will have on the network resulting in higher latency and lower bandwidth. Thankfully, by default, a router will not forward broadcast packets from one port to another.
A possible exam question related to what we've just learnt would give you a network diagram and then proceed to ask you to count the number of broadcast domains and collision domains. These sorts of questions should be very straight forward if you can remember the following rules:
- Hubs: One collision domain and one broadcast domain.
- Switches: Separate collision domain for each interface, one broadcast domain.
- Bridge: Separate collision domain for each interface, one broadcast domain.
- Routers: Separate collision domain and broadcast domain for each interface.
This beginning section of the first chapter finishes with one piece of wisdom, "The best network is one that's correctly configured to meet the business requirements of the company it serves" - something to keep in mind when designing networks and internetworks.
