These devices are essential elements of any wired or wireless network. It is therefore essential to understand how to manage the traffic and how to set paths to follow.
As the Router enter in the OSI model
Although only a theoretical framework, the OSI model is a good way to see how all these protocols, addresses and network devices (such as just the router) complement one another.
The Layer 2 and 3 of the OSI model are those that apply in this case. The Layer 2 Data Link layer is where there exist the Ethernet protocol, the MAC addresses and switches. Layer 3, network layer, is where there exist the IP protocol, IP addresses and routers. Consider that all traffic is sent from your computer, starting from Layer 7 (application of your network) up to Layer 1 (physical layer). Through the physical layer, medium traffic flows on your network (whether wired or wireless to a network).
The traffic passes through a router only if it is not on your local LAN. Routers operate at Layer 3 but above all must interpret at least 1 to 3 layers. Many parsed routers each type of traffic up to layer 4 to 7 and in various ways, but to simplify the concept we consider that only operate at Layer 3.
(network) because this is their primary function.
As routers use Ethernet, MAC addresses and IP addresses
As mentioned previously, the Layer 2 Ethernet protocol is the addressing system and the Ethernet or MAC address (also called physical address or Ethernet address). The Layer 3 protocol is IP and IP addressing system. Today, almost all the networks are based on Ethernet and IP. So, usually, each package on your network has an Ethernet MAC address source and destination and source IP address and destination. With a network protocol analyzer you could see the Ethernet MAC address source/destination and IP address source/destination.
What does a router with the network traffic?
Routers interpret these Ethernet and IP addresses, but are especially interested in the destination IP addresses that are sending the package. The router takes this destination (say both 63.248.129.2) and looks in its routing table. Here is the example of a routing table:
Location-A # show ip route
10.0.0.0/24 is subnetted, 2 subnets
R 10.2.2.0 [120 / 1] via 63.248.129.2, 00:00:16, Serial0
C 10.1.1.0 is directly connected, Ethernet0
63.0.0.0/30 is subnetted, 1 subnets
C 63.248.129.0 is directly connected, Serial0
Location-A #
The routes in the table are provided by static routes (entered by you) or dynamic courses. Using this table, the router tries to find the best path for your traffic, which in some cases may be unique. Often, this is a default route (also called gateway of last resort) that simply says: “If there are better paths to transmit this traffic, transmit here.”
Typically home users or small offices have a single Internet connection. In this case, have a default route and all traffic is forwarded to their Internet Service Provider (ISP). However, in the case of ISPs, there may be many devices for transmission of this traffic. Their routers must compare several hundreds of thousands of paths and select the one best for your traffic. This is done in a few milliseconds, and your traffic and return via the Internet may pass through hundreds of routers. But you appear almost instantly (the speed depends on many factors).
Looking for the best route
If you can not find a valid path for your traffic, the router discards your traffic and you send an ICMP “destination unreachable”. When the router is the best route and is ready to send your traffic, dealing with the following:
1. Making the Network Address Translation (NAT). This is not a traditional function of the router, but many routers today are running. This is especially true for those routers that run a small business and as all-in-one. Many companies have a dedicated firewall that also performs NAT. With NAT, your source IP address is translated into private source IP address public. If the router is making the PAT (NAT overload), the public source IP address is shared among many devices.
2. Replace your source MAC address with the MAC address of the router. Header of the data link is also added the MAC address of the destination host or router next.
3. Encapsulate the packet for the protocol of the WAN. Routers often perform protocol conversion. For example, suppose a router has a PPA T1 connection to the Internet and is connected to the LAN via Ethernet. The Ethernet frame must be de-encapsulated, modified, re-encapsulated in Ethernet, and PPP, before they can be transmitted through the PPA connection.
Other side of the link, the destination router performs all the same steps, but in reverse. This happens for every packet sent and any response received.
To see a real routing table produced by an ISP, you can make a telnet to the server public Cisco route. From here, you can do a show ip route and see what it looks like the routing table of the ISP.