146 Chapter 4: Bridges/Switches and LAN Design
To fully understand transparent bridging logic, consider Figure 4-6. A client first asks for a DNS
name resolution and then connects to a Web server. All three devices are on the same LAN
segment. ARP requests are used to find the MAC addresses of the DNS and the Web server.
Figure 4-6
Example Protocol Flows--Single Ethernet Segment
The following list provides some additional text relating the steps shown in Figure 4-6:
Step 1
The PC is preconfigured with the IP address of the DNS; it must
use ARP to find the DNS's MAC address.
Step 2
The DNS replies to the ARP request with its MAC address,
0200.2222.2222.
0200.3333.3333
Web
Server
Name
Server
DMAC = FFFF.FFFF.FFFF
SMAC = 0200.1111.1111
ARP (DNS)
0200.2222.2222
0200.1111.1111
1
DMAC = 0200.1111.1111
SMAC = 0200.2222.2222
ARP
2
DMAC = 0200.2222.2222
SMAC = 0200.1111.1111
DNS Request
3
DMAC = 0200.1111.1111
SMAC = 0200.2222.2222
DNS Reply
4
DMAC = FFFF.FFFF.FFFF
SMAC = 0200.1111.1111
ARP (Web)
5
DMAC = 0200.1111.1111
SMAC = 0200.3333.3333
ARP
6
DMAC = 0200.3333.3333
SMAC = 0200.1111.1111
Connect to Web
7
ch04.fm Page 146 Monday, March 20, 2000 5:02 PM