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Ch. 20 Q and A
CS332,Fall 2017Victor Norman
Switch/bridge vs. router
Q: Whatexactly is the difference between a switch, bridge, and router? I understand they both connect two or more arbitrary networks, so how exactly are theydifferent?A: Switches and bridges operate at layer ____.A router is at layer 3. So, it connects LANs.LANs can have different layer 2s. How could we communicate between machines on different layer 2s?Answer: a router that speaks both layer 2s, and somehow translates. But, the machines have to be able to identify each other and have a common “language”. So, layer 3 is that common language – IPv4 or IPv6.
Router vs. Switch
Q: Is a router basically a layer 3 switch?A: Not really.
Connect Networks at Layer 2?
Q: Can you connect networks withonly alayer 2 system?A: No. Layer 2 gets a packet across a network, not across multiple networks.
Universal Service
Means every computer can talk “directly” with every other one.A message is not addressed to an intermediate location, which would then have to re-label it for delivery.Q: What is the main difficulty with providing universal service?A: Scalability, I’d say. How to label every machine uniquely so every machine can send “directoly” to any other machine. And how to find these other machines?
Does a computer know a router is there?
Q: Inthe book, the author says that a network "treats a connection to a router the same as a connection to any other computer" (pg.337). Does this mean that one of the distinctions between a router and a switch is that the computer actually knows the router is there, whereas a computer does not know that a switch is there?A: Yes!! The computer will use the router to forward packets to networks on the other side of the router.
Home router vs router at ISP
Q: What is the difference between a home router and router at an ISP?A: First, scale. An ISP’s router can forward traffic from hundreds of businesses.Second, functionality. An ISP’s router implements many more protocols and they can be more finely tuned and monitored.
Alternatives to TCP/IP
Q: Are there alternatives to TCP/IP in use?A: No, not that I can find.There have been many alternatives over the years, but they have all died off.Because TCP/IP is best.
Unorganized internet
Q: I found it very interesting that the internet does not consist of a single centralized router. The figure on page 340 shows this well and I never realized that the internet is so "unorganized".Picture of Level3 network only.
Forwarding traffic
Q: How does a router know how to forward traffic for a remote destination?A: It consults its routing table.Q: How does the routing table get configured?A: You’ll have to keep coming back to find out…
Virtual Networks
Q: What is a virtual network?A: IP at layer 3 does not need to know about which layer 2 is being used to forward the packets across the networks. It just assumes the layer 2s can do it, as best as they can. Thus, it “virtualizes” layers 1 and 2. Networks differ at layer 2 and below. At layer 3 and above, they are all the same – IP and protocols above it.
Old Slides
Leased data circuit
Q: What is a “leased data circuit”?A: You can purchase a dedicated connection between two sites from your ISP. They will reserve resources on their routers (and on routers from companies they partner with) so that your traffic will flow smoothly from end-point to end-point.It is like a permanent VPN connection. Can be implemented with MPLS, which is a circuit-based layer 2.
Multiple locations on one network?
Q: Canyou answer in detail how a company with multiple locations maintains a one network illusion? I understand that they use internet routing but how does that work with their ISP?A: I’ll draw a picture.
Internet, internet, intranet
Q: Thebook refers to the internet and the Internet (capitalized). Can you go into more detail on the difference?A: The Internet is the global network where every machine must have a unique IP address. An internet is a local network (of connected LANs), e.g., inside a company.Often called an “intranet”.
Air Gap
Used to be that different networks had an “air gap”, which wasn’t desirable then.Now, for security some networks have an air gapthe network is not connected to any other network at all.Bad guys have tried tojump the air gapby listening to key strokes on one machine to hear passwords, etc., across the gap.
Representing networks in pictures
There may be confusion with picture on page 337.A cloud is used to represent routers and/or hosts connected together in a network.
Connecting networks
Q: Do routers need to be physically connected to each other to connect networks? If so, is there a cable going out to Hawaii and other islands like the cable that crosses the Atlantic? Is there a cable that goes under the Pacific too that connects the US to Asia?A: All right: videos, etc.Submarine cable map;sled;boat;repeaters, etc.(about 5 minutes in)
Router with Ethernet andWifi
Q: How does a router connect an Ethernet network to a Wi-Fi network? Is it mostly through software applications?A: It has an interface for each. A port for theethernetconnection, and an antenna for thewifi. Each interface can be configured.
Router size
Q: How many networks can a router connect? Is the limitation on the number of networks processor speed? And how actively are routers actually managed?A: Routers can have hundreds of interfaces and connect many thousands of networks on those interfaces. The limit is the memory and processor speeds.Routers are managed through network management tools, like HPOpenView. They use SNMP to get stats from the router periodically and use rules to determine if there is a problem.
Heterogeneous Networks
Q: What is it that makes a network heterogeneous? Do they mean different operating systems, or something else?A: The author means networks that use different Layer 2 protocols. E.g., my DSL modem/router at home talks over a virtual circuit to AT&Ts equipment, but in the house it is wired and wireless Ethernet. And, we have no idea how the data is carried in the “middle” of the Internet.
Virtual Network
Q: A virtual network seems like the perfect scenario, why don't people use these more often?A: The Internet is a virtual network (by some definition). It appears to be one big network, when really is a conglomeration of many networks, connected by routers.
Shared drives == Universal Service?
Q: So are the shared drives/networks that one can access on all computers (like the W:/Glacier drive) a part of universal service?A: That is not what the author means. He means all computers on the Internet can talk to each other. That does not mean they have the authorization to access services within companies’/schools’ networks.
Router definition
Q: What exactly is a router? Is it like a switch that is built to connect with any technology and is connected to another network?A: It operates at Layer 3, connecting multiple networks. These networks might use different layer 2s (Ethernet, ATM, ISDN, etc.). The router can move data between the networks if it has the correct port types and programming to send/receive on those layer 2.
Router definition (2)
Q: What is the difference between a router and a switch?A: A switch is at layer 2. A router at layer 3. A router gets a packet from a layer 2 interface. The software strips off the layer 2 header. Then the router software inspects the layer 3 header and figures out how to forward the packet – i.e., which outgoing interface to send the packet on. Then, the packet is re-encapsulated in that interface’s layer 2, and sent.
Router definition (3)
Q: Are routers like we use at home generally connected to one other network, or multiple? Are there centralized networks that many others are connected to?A: Routers at home are connected to 2 networks – one inside your house, and one outside. Your router gets an IP address (via DHCP) from your ISP’s router. Many homes are connected to each ISP’s router.
Router definition (4)
Q: Is the only difference between a router and a hub/switch that a router connects networks and a hub/switch connects hosts?A: Yes, exactly.
Multiple interface configuration
Q: Are router able to interconnect networks because their memory stores the different network technology types somehow?A: Yes! When you configure a router, you tell it what protocols will run on what interfaces.
Ratio of routers and networks
Q: (Summarized question). Can one router only connect 2 networks?A: No. Most (non-home) routers connect as many networks as they have ports. So, if you have 8 ports, you can route between 8 different networks. You would use one of the ports to “trunk” traffic to another router or to your ISP.
Routers / different languages
Q: So essentially routers are used to bridge networks of differing technologies, is this how different countries bridge networks of different languages?A: Routersconnectnetworks of differing technologies, via a common layer 3 (IP layer). The layer 2 can differ, but every machine speaks the common language, IP, at layer 3.
Q: What does the book mean when it says "Both hosts and routers need TCP/IP protocol software"?A: The whole has adopted IP as its de facto standard (i.e., that’s what everyone uses). So, if you want to play, you use IP. Services on the Internet are offered over TCP, typically. So, your machine better use TCP if you want to get any services. (A router only looks at IP, not TCP (layer 4), in general.)
Internet vs. internet
Q: Just to be clear, when people refer to “the Internet”, they are actually referring to their localized internet, and the “Internet” is actually a series of internets...?A: The Internet (capital I) is the global network of networks. A company can have an internet, sometimes called an “intranet”.
Q: So, in the TCP/IP model, the TCP layer (4) is basically used to address the security and reliability of the IP layer (3)?A: TCP provides security and reliability, correct. IP (layer 3) is still best effort – i.e., if it cannot forward a packet, it happily just discards it.





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ch 4 Calculs astronomiques et traçage -