Introduction To The OSI Reference Model - Part 2

By: Daniel Imbellino
Updated: Nov 18, 2013

Click here for part one

Layer 3: The Network layer handles routing of network transmissions (data), IP (Internet Protocol) addressing (logical addressing), quality of service, and many network management functions. The protocol data unit at this layer is the “Packet”. Routers work at the network layer by analyzing packets of data being sent and received through its logical ports. For instance ports 80 or 8080 are used for routing data to your web browser specifically. Packets have IP addresses affixed to them, such as 12.45.195.10, this is a public IP address, but there are also private IP addresses that are usually assigned by routers. 192.168.1.0 would be a typical address for a router, then the router would assign a block of IP addresses, say from 192.168.1.1 to 192.168.1.254. You can also access your routers management software directly by entering the routers IP address into any browsers main search bar. In the case of this network, if we were to type in the starting address of 192.168.1.0, this would take us to the routers management software where we could login and access its routing table, quality of service functions such as allowed bandwidth, limiting a number of concurrent connections, etc.

We could also route incoming data to a specific PC on your network by entering the PC’s IP address assigned by the router into its port configuration table and assigning a certain type of incoming data to be routed to that PC by port number. For instance, on my router I could forward all incoming remote desktop and remote assistance data and connections to one PC, such as my laptop by entering 3389 in my port configuration page, and assigning that port to only my laptop’s IP address. In this case, no other PC on my network would be able to initiate a remote desktop, or remote assistance session on my network. Routers add a level of security to networks by hiding your computers private IP address from the public internet, by port addressing, network wireless security protocols, and the integration of advanced firewalls. Besides routers, most switches nowadays work at the network layer.

How do I know what my routers IP address is in order to connect to it?

Well, on a Windows based PC, just click start, run, and type CMD in the run box and hit enter. Now type "ipconfig /all", without the quotes, and hit enter. Scroll to the top and look for the “Default Gateway”, on the right you will see the IP address of your router.

What if I can’t connect to the internet what do I do?

The first thing you should do is to check your PC’s logical connection, regardless if it’s a laptop or desktop PC. If your Desktop is connected by Ethernet, then check that the cable is firmly connected on both ends. Click start, type run, type CMD. Now type ipconfig /all, now look for your IPV4 address. If you see an IP address that starts with 169.254, then you know you are not connected to your router. This doesn’t necessarily mean your network is down, it just means there’s an issue with communication to your router specifically. The 169.254 IP address is what we call a APIPA or “Automatic Private IP Address”. Windows based operating systems automatically assign this type of address when a network interface card (wired or wireless) is unable to make a connection to the network by contacting a DHCP server for internet access. At this point you should check the network and connection indicators on your router. Try resetting your router by unplugging all cables including the power cable for 60 seconds. This will wipe all cached data off the router. Now plug it back in and try to reconnect to the network.

If your internet connection is intermittent, this may indicate a failing router, modem, or bad Ethernet cable. The locking mechanisms on Ethernet cables lose their strength over time and should be replaced yearly, especially in an office setting. Routers and modems are devices that never turn off! They are always on, and so are the subject of constant heat and are subject to oxidation. Oxidation causes the circuits to lose their electrical conductivity, and this is what causes their intermittent shorts (one moment they work, and the next they don’t). It’s best to just replace the router when all else fails. You can also assign static (permanent) IP addresses to computers on an internal network with a router, by filtering addresses based on a specific PC’s MAC address.

Layer 2: The Data Link Layer handles raw data signals entering thru or being sent from a computers network interface card (NIC). NIC drivers operate at this layer. The protocol data unit at this layer is called a frame. The frame trailer consists of a “Frame Check Sequence”. In the frame check sequence exists a CRC or “Cyclical Redundancy Check value, which calculates data using a mathematical function to validate that the data sent is identical to the data received. The CRC is recalculated when data arrives at its destination NIC, and if the values match, the data is believed to be accurate, if not, then the frame or packet is discarded and re-requested from the source. While network interface cards tend to read all frames that enter their interface, the NIC will pass a frame (packet) up through the preceding layers if the address in the frame matches the address of the computer (IP Address) or the address of the network interface card (MAC address). You can set some network interface cards to read all frames that enter its interface by setting the NIC’s frame setting to “Promiscuous Mode”. Not all network interface cards support this.

Layer 1: The Physical Layer handles the actual electrical or optical binary encoding of data that is to be transmitted across a network. This layer operates upon the hardware media that is used for transmission, this includes cabling, connectors, repeaters, hubs, etc. There are many encoding schemes in place today. One way to encode binary data as an example would be to send 1’s as a positive voltage, and 0’s as a negative voltage. Data may also be transmitted as optical light for high bandwidth situations.

Back to part one of the intro to the OSI reference model