Data Communication Concepts Part 5
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Computer A compares its IP address to Computer B's IP address using the subnet mask. They have matching network IDs, so Computer A sends an ARP broadcast to determine Computer B's MAC address. Computer B responds with its MAC address, allowing communication. When sending to Computer C, their network IDs do not match, so Computer A sends the packet to the default gateway instead of directly to Computer C. Subnet masks in binary and dotted decimal format are used to determine if the IP addresses have matching network IDs.
Data Communication Concepts Part 5
- 3. Computer A compares its IP address to Computer B’s IP address using the subnet mask, as shown in Figure 7.16. For clarity, I’ve added a line to show you where the ones end and the zeroes begin in the subnet mask. Computers certainly don’t need the pretty red line! Ah ha! Computer A’s and Computer B’s network ID match! It’s a local call. Knowing this, Computer A can now send out an ARP broadcast, as shown in Figure 7.17, to deter mine Computer B’s MAC address. The Address Resolution Protocol (ARP) is how TCP/IP networks figure out the MACaddresses based on the destination IP address.
- 5. Computer B responds to the ARP by sending Computer A an ARP response (Figure 7.19). Once Computer A has Computer B’s MAC address, it will now start sending packets. But what happens when Computer A wants to send a packet to Computer C? First, Computer A compares Computer C’s IP address to its own using the subnet mask (Figure 7.20). It sees that the IP addresses do not match in the ones part of the subnet mask—the network IDs don’t match—meaning this is a long- distance call.
- 7. Whenever a computer wants to send to an IP address on another LAN, it knows to send the packet to the default gateway. It still sends out an ARP, but this time to the default gateway (Figure 7.21). Once Computer A gets the default gateway’s MAC address, it then begins to send packets. Subnet masks are represented in dotted decimal just like IP addresses —just remember that both are really 32-bit binary numbers. All of the following (shown in both binary and dotted decimal formats) can be subnet masks: