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2

Literally? No. Unless the devices were very close, it's very unlikely they would ever have been directly connected. Even 20-30 years ago, in the era of T1's, there were repeaters, digital cross-connects, and multiplexing into larger T-carriers. As Ron has said, today the T1 is a relic. It will be emulated and carried as packets like every thing else "...


3

No. There may be a dedicated circuit between the routers and the nearest telco central office, but between offices they are switched and multiplexed onto other, higher capacity circuits. Today, most T1 circuits are emulated over a packet-switched (IP) network. They are rapidly becoming obsolete, and are being replaced by SIP over the IP network.


1

CSMA is used in both Ethernet (CSMA/CD) and Wi-Fi (CSMA/CA). Every device that uses one or the other uses it. It's almost always implemented in hardware on the network interface.


0

CSMA is for a shared medium. A host senses the carrier to determine if it is clear before sending. This is required for something like Wi-Fi, where the medium is shared, otherwise all hosts would just send, stepping on each other, resulting in garbled signals. It is still used for network protocols used on a shared medium.


1

How does my computer and x.x.x.x know how to talk to each other? They don't really. But they know who to ask. Routing is a collective effort. Most routers gateways only know a small number of other routes and gateways. Only few routers need to know all routes. A greatly simplified view: Your computer sees by the destination IP that the destination is ...


2

How does my computer and x.x.x.x know how to talk to each other? If I use a human messengers analogy, I would tell a person to go get me a package, and I would give him the address of the location and a map (he probably already have one). Exactly, an IP packet has both the source and destination IP addresses on it, much like the envelope of a letter. You ...


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Some devices and configuration methods offer more control over the inner workings of protocols than others. More control requires more effort.


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(This will tread on "historical trivia") The tiered construct was the original vision of the ARPANet (what became the internet.) The modern internet does not work like that. In fact, the early internet -- and ARPANet before it -- didn't entirely work like that either. With the advent of BGP, any network could be anywhere, connected to anyone. ...


4

There's not really a hierarchy. It's really more of a web. There are national (or international) ISPs that connect to a large number of other ISPs. There are smaller ISPs that connect to only a few other ISPs. Then there are large companies that have their own backbone (Google, Microsoft, Amazon, Facebook, etc). There's no "rule" about who can ...


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The security options from RFC 791 are long defunct. I don't think they were ever used outside of DoD (if at all).


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