/me tries to make sense of your questions...
(disclaimer: answers are given based upon my knowledge of TCP/IP and may be wrong if you use NETBIEU (sp?) or IPX as your ethernet protocol)
"I understand, if it's routed, the beginning of the packet is the source MAC to dst MAC with is the next router to get to the desired dest IP specified."
MAC addresses are mostly important when you're using hubs to route packets. You setup your network and you CAN program some hubs to route packets based upon MAC addresses. A MAC address should always be unique in a network setting so that they can represent that computer properly. MAC addresses are derived from the network card. Since sometimes you may get a conflict where more than one card has the same MAC address, depending on the card, you can change it. Some people believe that all MAC addresses are unique and cannot be changed. Don't listen to them. In a packet: SRC MAC = sending computer, DST MAC = destination computer.
"ok now, how does it work on the internet, with ppl who don't have a network card, or who connect to the net with a modem for example."
The internet doesn't use MAC addresses like that. Instead we've got the TCP/IP system. In that system, each computer is represented by a 32-bit number (IP). Routing is possible because there are routing tables that are passed around amongst routers to let them know where packets go. If a router doesn't know where a packet should go, they send it to a router that might know. Eventually the packet will make it to it's destination, or if it never gets there, a ICMP error response is sent back to the sender.
"What is the source MAC?? is it that 45-44-00-00 or smtg ...corespondance i get while scanning winBOXes..."
The source MAC is simply the address programmed into your network card.
For information about your ethernet card
type this into your command prompt:
ipconfig /all | more
The Physical Address is my MAC address for an adapter. It will look something like '00-C0-F0-78-30-CD'
The MAC address coming from a modem user will be a MAC address of the computer the user is dialed into.
"And do we broadcast?"
uh, broadcasting relates to UDP datagrams which gets sent across an entire submask.
"Does everyone on the subnet use FF-FF-FF-FF-FF-FF destination or use the network's router [email protected]
For broadcasting? FF-FF-FF-FF-FF-FF always.
"And how would one go about sniffing on smtg else than ethernet?"
What is smtg? I just might be unfamiliar with the acronym. But to give a generalized response... There are two different kinds of sniffers. There's a 'Packet Sniffer' which will log data being sent to and from your computer. Then there's a 'Ethernet Sniffer' which is only useful on networks where you don't have switching and can therefor ALSO log information sent between other computers on that network.