Subnetting

1
Subnetting

• 129.123.7.55 AND 255.255.255.0 129.123.7.0
• 129.123.7.148 AND 255.255.255.0129.123.7.0
• 129.123.7.0 COMPARED (MINUS) TO 129.123.7.0
  0.0.0.0
• 129.123.25.77 AND 255.255.255.0129.123.25.0
• 129.123.7.0 COMPARED (MINUS) TO
  129.123.25.00.0.18.0
• Since there is a non-zero result, the request
  must be forwarded to the routed via the gateway
  address.

2
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)

3
Link Layer

• Ethernet
• PPP
• Token rings

4
Link Layer Terms

• Framing
• Link Access
• Reliable Delivery
• Flow Control
• Error Detection
• Error Correction
• Half and Full Duplex

5
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

6
Error Detection

• Parity Checks (16 bit example)
• Data bits Parity
• 0111000110101011 1
• One bit even parity
• Cant tell which bit is wrong

7
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

8
Checksumming (CRC)

• D . 2r XOR R D of data bits,
• r CRC bits
• Polynomial Code
• See example on page 429 in book.

9
Multiple Access methods

• Shared Wire (ethernet)
• Shared Wireless (WiFi, 802.11a/b/g)
• Satellite
• Cocktail party

10
Multiplexing

• Time division
• Each packet has a defined time slot
• Requires precise timing
• Frequency division
• Each packet has its own frequency allocation
• Requires large bandwidth for many channels
• CDMA (Code division multiple access)
• Assigned codes (wireless techniques, Ch. 6)

11
CSMA (carrier sense multiple access)

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

12
Packet Traffic
Network Cable
13
Collisions
5
Signal 1
0
5
Signal 2
0
10
Sum of 1 and 2
0
Ethernet threshold
14
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 ffffffffffff

15
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

16
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).

17
DHCP Interchange
18
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

19
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)

20
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.

21
RJ45 Connector
1 2 3 4 5 6 7 8
Looking from the CABLE end
22
1
White/Orange
2
Orange/White
3
White/Green
4
Blue/White
White/Blue
5
6
Green/White
7
White/Brown
8
Brown/White
23
Ethernet Twist (hub-hub)
1
W/O
1
transmit
2
transmit
O/W
2
3
G/W
3
6
receive
W/G
receive
6
This can be used to connect two pcs together
24
Hubs and Repeaters

• Hubs (Star topology)

Twisted Pair
Backbone
Fiber, Coax, Twisted Pair
(Twisted Pair may need a twist!)
25
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!

26
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27
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28
Coaxial Installation
Backbone

• Multidrop (Bus topology)

T
Tterminator (50 ohm resistor)
Other computers
T
T
T
Repeater
29
Repeater Rule 10MB

• 3 repeater rule

Sys3
Sys 2
System 1
Repeater
Repeater
Repeater
Repeater
System 1 can talk to Sys 2 but not Sys3
30
Network Diameter (100MB)

• 205 Meter Limit

100M
100M
5M
10MB diameter is 500M
31
Switches and Bridges
Bridge
32
Bridge Insides
LAN B
LAN A
Filter based on ethernet address
Interface
Interface
33
Switch Insides
A
F
B
C
G
D
Switching based on ethernet address
E
34
100baseT to 10baseTHub/Bridge
10baseT
100baseT
100baseT to 10baseT Bridge Module
100baseT hub
10baseT hub
35
10MB Ethernet Rules

• 3 repeater (populated) limit
• 5 repeater per segment limit
• 7 bridges per ethernet lan
• Cable lengths
• 1024 stations per ethernet lan

36
How to get around limits?
Hubs
Switch
ROUTER
Firewall
To the Internet
(switching based on IP address)
Other Lans
37
Ethernet Errors

• Carrier Loss
• Link Lights?
• Collisions
• What does the collision light indicate?
• Troubleshooting tools
• tcpdump, snoop, traceroute, ping

38
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

39
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 its 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.

40
Token Ring
4 or 16 MB/s
Mic (Media Interface Connector at each computer)
41
MAU

• Multistation Access Unit

Ring in
Ring out
Computer Ports
42
PPP Protocol

• Modem Pools
• Multiple Network Layer Protocols
• Multiple types of links
• Error Detection
• Failure detection (liveness)
• Address negotiation
• Simple

43
Link Layer
The End
REMEMBER TEST NEXT WEEK!