Ch 20 Q and A IS333, Spring 2014 Victor Norman
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: Routers connect networks 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.
TCP/IP 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.