Hi, Can anybody tell me why MAC addresses are unique when they are supposed to be used within a LAN?. Are MAC addresses transmitted across internet also?. If for security reason MAC address has to be known to government agency or service provider then how do they find it from a remote system when only IP packets are supposed to be routed by routers? Thanks Sharma M.C.L

If you are using Ethernet then your ISP needs to know your MAC address. They don't find it, you tell them. The TCP/IP protocol is encapsulated within the Ethernet protocol containing the MAC address. The ISPs hardware will strip out the Ethernet protocol when it has done with it, or in my case, the modem will as I am using PPPoATM. The modem will also strip the Ethernet protocol from incoming packets before sending it to the router. The Router will then apply NAT re-encapsulate it sends it to the switch sends it to the necessary device. All a MAC address does is identify a particular network device, in the case of an ISP, your modem. What use a MAC address would have to Government Security Services is doubtful as it can be so easily changed simply by using a different network device. They are unique because if you had two NICs in the same network with the same MAC address it you would have sever problems. People have enough problems with duplicate IP addresses on a network. Imagine the problems with duplicate MAC addresses as well. Stuart

A mac is the most basic part of any network (these days). Think of it as your address or phone at your home. What would happen to communications? Yes and no or depends on what you mean. One uses APR tools and caches to translate a mac address. A mac is not a security feature. In fact you will have many adapters with the same mac address. Normally only large companies might ever find two that are the same. (unless one spoofed it) I read it wrong and answer it wrong too. So get off my case you goober.

>> In fact you will have many adapters with the same mac address. Normally only large companies might ever find two that are the same. (unless one spoofed it) << Not quite true. MAC addresses are gobally unique - unless they have been spooffed or cloned. The first three octets of a MAC address identify the the manufacturer. Each manufacture has their own code. The last three octets is the number allocated by the manufacturer. That gives each manufacturer a possible 16,777,215 addresses to chose from. I don't think even Cisco have made anywhere near 16,777,215 network devices. If they ever do all they need do is get themselves another identifier and start again, there are plenty to go round. Allied Telesyn has five of them and there certainly aren't 16,777,215 manufactures making network devices. MAC Identifier Stuart

"Not quite true. MAC addresses are gobally unique - unless they have been spooffed or cloned." I only state that because that was taught at a cisco class. They could have been wrong. I tend to believe them as even with a Cisco switch it tends to be much more than 1 port per equipment. In fact any possible IP assignment per device would require a mac. That would raise the number of mac's used much more than one would start out to think. Then you consider that they use a range and not a random pattern and it lowers that number. I read it wrong and answer it wrong too. So get off my case you goober.

>> In fact any possible IP assignment per device would require a mac. That would raise the number of mac's used much more than one would start out to think. Where did you get that idea? Certainly not from cisco. You don't get a seperate mac address for each IP address assigned to an interface. An interface only has 1 mac address, but could be assigned many IP addresses. For example, I just assigned a 2nd address to my system and here's the config. Connection-specific DNS Suffix . : Description . . . . . . . . . . . : SMC EZ Card 10/100 (SMC1211TX) Physical Address. . . . . . . . . : 00-E0-29-6F-09-1D DHCP Enabled. . . . . . . . . . . : No IP Address. . . . . . . . . . . . : 192.168.1.25 Subnet Mask . . . . . . . . . . . : 255.255.255.0 IP Address. . . . . . . . . . . . : 10.100.0.118 Subnet Mask . . . . . . . . . . . : 255.255.248.0 Default Gateway . . . . . . . . . : 10.100.0.1 DNS Servers . . . . . . . . . . . : 10.200.1.15 10.200.1.20 =====================================================