Now that we have the basics of networking theory, what does this mean to network functions and devices? Lets start at the bottom of the OSI layer and work up.
The physical layer is exactly that – the physical (or wireless) connectivity between devices. The physical layer includes the network interface cards (NIC) and cabling between the devices. The most common cabling in use today Twisted Pair and is defined in various categories based on the number of twists per inch in the cable; the more twists the less interference by external electronics. Most common ratings are category (or CAT for short) 3, 5 and 6, with some “enhanced” versions available for higher performance.
- CAT 3 is traditionally used for phone lines and is capable of transmitting up to 10Mb/sec
- CAT 5 is the most commonly used networking cable and is capable of transmitting up to 100Mb/sec
- CAT 6 is fairly new and capable of transmitting up to 1Gb/sec
Other physical mediums include coaxial (not commonly used in a datacenter), fiber (used for high-speed and long distances), and wireless.
Network devices at the physical layer include: hub/bridge/repeater – an unintelligent device that rebroadcasts/repeats signals to all available ports. No addressing or routing is performed and collision of packets from multiple devices can occur frequently.
The datalink layer is the physical addressing of the devices on a network. Since several devices can share the same physical connection, the datalink layer utilizes the MAC address of each device to direct traffic. This is done by the hardware built into the NIC to only pickup traffic targeted for its address, thereby minimizing processor overhead on the recieving systems.
While the MAC address is normally hard-coded in a NIC, most devices today allow for a different MAC address to be configured via software. This is useful when two NICs on the same network have the same MAC address (supposedly shouldnt happen, but I have seen it occur), or when you need to pretend your a different device (sometimes referred to as spoofing or promiscuous mode).
The format of the MAC address follows a standardized format – a 12 digit hexadecimal number split into 2 parts. The first 6 digits identify the manufacturer of the network card (use http://coffer.com/mac_find/ to lookup the vendor), and the remaining 6 digits are randomly created. More information on MAC addresses can be found at http://en.wikipedia.org/wiki/MAC_address.
Network devices at the datalink layer include: switch – a somewhat intelligent device that learns the addresses of devices around it, then inspects traffic to direct it only to the appropriate target. Once a connection between two devices is setup within a switch, unhindered communication can occur between the devices.
The network layer is where logical addressing of occurs, on top of the physical addressing of the datalink layer. This is where the IP address comes into play to allow a human defined separation and routing of traffic between departments, offices, or cities.
IP addressing follows a strict format defined by the IETF (Internet Engineering Task Force). The IP address is composed of 4 octets (or 32 binary digits), separated into 2 sub addresses: the network address and the machine address. The network address can be likened to the city/state/zip your mail is delivered to – a message can be sent from anywhere in the world and this unique address will deliver it to your local post office. The machine address can then be likened to the physical address of your home, once the local post office has the mail, it can deliver it to your location.
The size of the network and machine address however are not static, their size can vary from network to network based on the administrators design. This is where the subnet mask comes into play, it tells the devices how much of the IP address is the network address and how much is the machine address. The subnet mask can define as little as 1 binary digit for the network address and as many as 31, and the same goes for the machine address. More information about the history and design of IP addressing can be http://en.wikipedia.org/wiki/IPv4
Network devices at the network layer include: router – a more intelligent device that has a set of rules to direct traffic between networks. This device is not aware of the individual devices that exist on each network, but instead only about the topology of the network addresses from its point of view.
Up next – Routing…