Networking Reference
In-Depth Information
Chapter 3 introduced MAC addresses . As a reminder, a MAC address is a 48-bit address expressed in a
hexadecimal format. For example, a MAC address looks like 00-23-5A-33-C4-CA.
Four bits are used to represent each hexadecimal character. Chapter 6 covered hexadecimal
numbers in more depth.
Every network interface card (NIC) has a MAC address assigned to it. MAC addresses are typically
burned into the card and unchangeable, though some NICs allow you to modify the MAC. Additionally,
when a computer sends data to another computer, it always includes both its own IP address and its own
MAC address as part of the source information.
A simple switch starts with very little knowledge when it's turned on. It knows what ports it has, but it
does not know which computers are connected to which ports. However, as traffic is sent through the switch,
it learns. It populates an internal MAC address table with the MAC addresses of each computer and maps
them to the port to which they're connected.
Advanced switches (managed switches) are configurable. You can configure them with the
MAC addresses of connected computers.
Consider Figure 8-7 . It shows a four-port switch with a computer connected to each port, and it shows
their MAC addresses. Imagine this switch is just turned on. When PC-1 sends data to PC-3, the switch
doesn't know what port PC-3 is on, so it sends the data to all ports. However, the packet from PC-1 includes
the MAC address of PC-1. The switch silently says “gotcha” and starts populating the MAC table by log-
ging port number F0 with the MAC address of PC-1.
Figure 8-7: Mapping ports to MAC addresses
When PC-3 answers, it includes the destination MAC address of PC-1 and the source address of PC-3.
Again, the switch silently says “gotcha” and logs the MAC address of PC-3 with port F2 in the MAC address
table. Since the switch knows that PC-1 is connected to port F0 (based on the MAC address), it internally
switches the data from PC-3 to PC-1 on port F0. In a very short period, the switch will learn the MAC ad-
dresses of each computer along with their associated ports.
When the switch maps the MAC addresses to ports as traffic passes, the table is updated dynamically.
However, an administrator can configure a managed switch with specific MAC addresses as static entries.
Dynamic entries can be overwritten as time passes, but static entries remain.
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