Ch. 20 Q and A
CS332, Spring 2016
Victor Norman
Switch/bridge vs. router
Q: What exactly is the difference between a switch,
bridge, and router? I understand they both connect two
or more arbitrary networks, so how exactly are they
different?
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.
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?
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.
Internet, internet, intranet
Q: The book 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”.
Multiple locations on one network?
Q: Can you 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.
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.
Does a computer know a router is
there?
Q: In the 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.
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.
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…
Old Slides
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
gap
– the network is not connected to any other
network at all.
• Bad guys have tried to jump the air gap by
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 and Wifi
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 the
ethernet connection, and an antenna for the
wifi. 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 HP OpenView. They use SNMP to get stats
from the router periodically and use rules to determine if
there is a problem.
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.
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.