Routers are core to broadcast networks. So, I want to introduce some fundamental concepts regarding networking that may help you understand how routers work.
Ethernet is both a protocol and a hardware specification. The Ethernet specification details electrical signals and voltages on the wire (or the RF transmission scheme in the case of wireless), and it lays out how Ethernet packets are built and what information Ethernet headers contain. Headers contain source and destination addresses. This helps routers direct packets to destination. Ethernet addresses differ from IP addresses.
Ethernet addresses (sometimes called MAC addresses) appear as: nn:nn:nn:nn:nn:nn (six groups of two hex digits). These are hardcoded into network interface chips on a specific computer and typically cannot be changed. Think of MAC addresses as Vehicle Identification Numbers stamped into engine blocks of cars. Ethernet is defined by the IEEE in its 802-x family of standards.
IP is a core protocol. Its job is to move datagrams from one device to another. (A datagram is defined as a self-contained, independent entity of data. It carries sufficient information to be routed from a source to the destination computer without reliance on earlier exchanges between the source and destination computer and transporting network.) The IP layer prepares data sent to it by applications or other higher protocols for transmission across a specific network (an IP network), taking into account things such as packet length, hardware addressing structure and how data should split across multiple packets.
IP packets are the payload carried in Ethernet packets in an “IP over Ethernet” network (about 95 percent of all network installations). MAC addresses may be hardcoded, but IP addresses can be changed in the network configuration menu of almost all modern operating systems. IP is defined by the Internet Engineering Task Force (IETF) as Internet Standard 5.
Once a physical network is in place (wires, connectors, switches, etc.), equipment can be connected to it. With Ethernet and IP, the capability exists to identify specific equipment as packet sources and destinations. And, the means are available to logically group equipment into networks and pass messages from one network to another. But, another protocol is critical to the network functioning.
Here is a question: What mechanism associates the MAC address and, therefore, a specific physical piece of hardware, with an IP address?
Address Resolution Protocol, or ARP, provides the solution. Figure 1 shows an actual ARP transaction captured using Wireshark — a free packet capture and inspection tool. Assume a router with an IP address of 192.168.1.1 needs to send a packet to 192.168.1.43. The router will send an ARP request asking, “Who has 192.168.1.43? Tell 19.2168.1.1.” The computer with that IP address responds, “192.168.1.43 at 00:15:53:7C:22:5C.” The router then knows where the packet should go. In most cases, once the router has this information, it stores it in its ARP table for memory. ARP is defined in IETF Internet Standard 37.
Now that the IP address to MAC address question has been resolved, the network is almost ready to go. Packets can be sent from one computer to another on the same network, but what about from one network to another?