Subnetting

129.123.7.55 AND 255.255.255.0= 129.123.7.0

129.123.7.148 AND 255.255.255.0=129.123.7.0

129.123.7.0 COMPARED (MINUS) TO 129.123.7.0= 0.0.0.0

129.123.25.77 AND 255.255.255.0=129.123.25.0

129.123.7.0 COMPARED (MINUS) TO 129.123.25.0=0.0.18.0

Since there is a non-zero result, the request must be forwarded to the routed via the gateway address.

Subnetting continued

Address range Subnet Mask Gateway

129.123.51.1-62 255.255.255.192 129.123.51.63

129.123.51.65-125 255.255.255.192 129.123.51.126

129.123.51.129-189 255.255.255.192 129.123.51.190

129.123.51.193-253 255.255.255.192 129.123.51.254

129.123.51.50 AND 255.255.255.192 = 129.123.51.0

129.123.51.70 AND 255.255.255.192 = 129.123.51.64

COMARISON: ABS(0 – 64) = 64, go to router (gateway)

Link Layer

• Ethernet

• PPP

• Token rings

Link Layer Terms

• Framing

• Link Access

• Reliable Delivery

• Flow Control

• Error Detection

• Error Correction

• Half and Full Duplex

Ethernet Hardware

• Network Interface Cards• Physical Link

– Twisted Pair• 100 Meter length limit

• Point to Point

– Fiber• Expensive

• Point to Point

• 2000 meter length limit

Error Detection

• Parity Checks (16 bit example)

• Data bits: Parity

• 0111000110101011 1

• One bit even parity

• Can’t tell which bit is wrong

Two dimensional Even Parity

10101 1 10101 1

11110 0 10110 0

01110 1 01110 1

00101 0 00101 0

Parity Ok Parity Error

Forward Error Correction possible

Checksumming (CRC)

D . 2r XOR R D=# of data bits,

r = CRC bits

Polynomial Code

See example on page 429 in book.

Multiple Access methods

• Shared Wire (ethernet)

• Shared Wireless (WiFi, 802.11a/b/g)

• Satellite

• Cocktail party

Multiplexing

• Time division– Each packet has a defined time slot– Requires precise timing

• Frequency division– Each packet has it’s own frequency allocation– Requires large bandwidth for many channels

• CDMA (Code division multiple access)– Assigned codes (wireless techniques, Ch. 6)

CSMA (carrier sense multiple access)

• Listen before speaking

• If someone else begins talking at the same time then stop talking (collision detection).

Packet Traffic

Network Cable

Collisions

Signal 1

Signal 2

0

50

5

0

10

Sum of 1 and 2

Ethernet threshold

MAC (Media access control addressing)

• 12 digit hexadecimal address

• Unique to every system on the LAN

• Usually hard coded into the NIC but can be changed in software

• Broadcast address ff:ff:ff:ff:ff:ff

Address Resolution Protocol (ARP)

• Translates IP addresses to MAC addresses.

• Keeps in subnet traffic inside the subnet.

• Uses timers to clear ARP tables

• arp –a lists arp entries (PC or Unix)

• arp –d deletes an arp entry

DHCP

• Dynamic Host Configuration Protocol

• Configure many machines from one server at boot.

• Multiple servers, uses router helper addresses to discover servers.

• Addresses can be static (same address for every lease period) or Dynamic (different address for each lease period).

DHCP Interchange

Ethernet Cabling Cont.

– Coax (10 MB ethernet only)• obsolete

• inexpensive

• Bus System (party line)

• Thin (185 Meter length limit)– 30 connections/cable, min 18 inches between

• Thick (500 meter length limit)– 200 connections, min 2.5M between

• Transceivers

10baseT/100baseT

• 2 twisted pairs

• Transmit/Receive

• 100 meter/328 foot distance limit

• Uses standard telephone modular plugs

• Category 3 (10mb) vs. Category 5 (100mb)

Twisted Pair Ethernet

• Ethernet uses wires 1 and 2 for one pair and 3 and 6 for the other pair.

• Wires 4 and 5 are used for analog (single pair) telephone

• Ethernet data is transmitted on one pair and received on the other pair in the cable.

RJ45 Connector1 2 3 4 5 6 7 8

Looking from the CABLE end

1

2

3

4

5

7

8

6

White/Orange

Orange/White

White/Green

Green/White

Blue/WhiteWhite/Blue

White/Brown

Brown/White

Ethernet Twist (hub-hub)

W/O

O/W

G/W

W/G

1

2

3

6

1

2

3

6

transmit

receive

transmit

receive

This can be used to connect two pc’s together

Hubs and Repeaters

• Hubs (Star topology)

Twisted Pair

Fiber, Coax, Twisted Pair

Backbone

(Twisted Pair may needa twist!)

Hub Advantages

• Standard Telephone Wiring

• Standard Punch Blocks and Cross Connects

• Ability to disable a single port

• Easier to monitor traffic patterns

• Visible Indication of Link Status

• If a wire is cut only one port is affected – Unless the backbone feed is cut!

10/100 Switch

10BaseT hubs

Telephoneconnections

Coaxial Installation

• Multidrop (Bus topology)

T

T=terminator(50 ohm resistor)

Repeater

Backbone

Other computersTTT

Repeater Rule 10MB

• 3 repeater rule

Repeater

Repeater

Repeater

Repeater

System 1

Sys 2 Sys3

System 1 can talk to Sys 2 but not Sys3

Network Diameter (100MB)

• 205 Meter Limit

Repeater

Repeater

100M

5M

100M

10MB diameter is 500M

Switches and Bridges

Repeater

Repeater

Repeater

RepeaterB

ridge

Bridge Insides

Interface

Interface

Filter based on ethernet address

LAN A LAN B

Switch Insides

A

B

C

D

E

F

G

Switching based onethernet address

100baseT to 10baseTHub/Bridge10baseT 100baseT

10baseT hub

100baseT hub

100baseTto10baseTBridgeModule

10MB Ethernet Rules

• 3 repeater (populated) limit

• 5 repeater per segment limit

• 7 bridges per ethernet lan

• Cable lengths

• 1024 stations per ethernet lan

How to get around limits?Hubs

Switch

ROUTER

Other Lans

To the Internet(switchingbased onIP address)

Firew

all

Ethernet Errors

• Carrier Loss– Link Lights?

• Collisions– What does the collision light indicate?

• Troubleshooting tools– tcpdump, snoop, traceroute, ping

Routed vs. Switched Network

• Routed– IP/IPX Layer Routing

– Subnetted Network

– Broadcast Filtering

– More Expensive

– Complex Configuration

– Better Control

• Switched– Ethernet Layer Routing

– Flat Network

– Broadcasts propagate

– Less Expensive

– Simple Configuration

– Loose Control

– Network Diameter limit

Token Ring

• Special packets called tokens circulate in a ring from computer to computer.– If a free token is detected then a computer will

take control and send it’s data– If a token is destined for a specific computer

that computer takes the token– One one computer at a time can use the token

so no collisions take place.

Token Ring

Mic (Media Interface Connector at eachcomputer)

4 or 16 MB/s

MAU

• Multistation Access Unit

``Ring in Ring out

Computer Ports

PPP Protocol

• Modem Pools

• Multiple Network Layer Protocols

• Multiple types of links

• Error Detection

• Failure detection (liveness)

• Address negotiation

• Simple

Link Layer

The EndREMEMBER TEST NEXT WEEK!