So what is a network?
A network supports the interconnection of many devices and a protocol for ensuring they can communicate with one another in the most effective way. The best way to understand how networks work is to visualise each element of the network as a layer, one on top of another. The conceptual model that describes this layered model is known as the OSI Reference Model, which has seven layers. All Network professionals use this in their day to day design and engineering work.
For our purposes we can simplify it into three layers, going from the bottom up there is:
1. The Physical layer – the cable between machines (typically called 100BaseT, uses four pairs), the card in the back of your machine (802.3 Ethernet NIC) or Wireless Networking adapter (802.11n etc) etc.
2. Data Link, Network and Transport Layers – Responsible for managing the addressing, routing and packaging of data around the network. Includes the Internet protocol (TCP/IP), gaming and file transfer based protocols (such as UDP), and VPN networking from home to your office (PPTP or IPSec)
3. And the Session, Presentation and Application Layers – file sharing and database access in the office (NetBIOS, Named Pipes, NFS), Internet browsing (HTTP, DNS), eMail (MIME, SMTP, POP3) and securing Internet banking or shopping (SSL/TLS)
Breaking it out into layers like this helps us understand when we buy software or hardware which layer(s) it works at and therefore what it can provide for us. Is it providing connectivity, access to another Wide Area Network (WAN, i.e. for the Internet), security or access to my own Local Area Network (LAN, i.e. for access to a printer). It’s extremely useful when diagnosing problems with networks.
In order to send a letter to your friend in the next town, or a country on the other side of the world the postal system requires an address which hones down through the address layers from country to house number to narrow down exactly where your friend lives and which post box the postman should drop the letter in. Digital Networks work in a similar way only rather than moving mail around they move digital data packets.
MAC addresses – The Media Access Control address identifies a single piece of hardware on the physical network and is a scheme with a long number designed to be globally unique. It’s set in the hardware at the point of manufacture. An example of a MAC address is 1A-2F-1D-9C-7A-3C (Layer 2).
IP address – The Internet Protocol address uniquely identifies all network interfaces that are typically endpoints on the Internet, or your own local IP based network (in an office). Within the local or global (Internet) address space the IP address must be unique, otherwise the IP routing protocol won’t know where to send the packet. An IP address will be mapped across to a physical MAC address as described above, the mapping is held in the routing tables of router hardware on the network. A sample IP address is 192.168.0.1 (Layer 4). Addresses are divided up into Class A, B and C each having a larger address space for larger organisations requirements. Small offices and domestic addresses are almost always Class C and the 192.168.0.nnn network is reserved for anyone to use on any small private network.
DNS name (and server) – Domain Name Server based addresses are just names, with dots to designate levels of uniqueness. We are all familiar with Internet domain names such as; www.cryopc.co.uk, the DNS name in this instance is the ‘cryopc’ element of the full address. DNS servers store all these hostnames and the IP addresses they map to.