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LAN Concepts

Version C.01

H3065S Module 1 Slides


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2001 Hewlett-Packard CompanyH3065S C.01 2

What Is a Network?

A Network is a series of devices interconnected by communication pathways. Local Area Networks (LANs) span relatively small geographic areas.Wide Area Networks (WANs) span relatively large geographic areas.

Chicago Office LAN Tokyo Office LAN

Boston Office LAN

WAN


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The OSI Model in a Nutshell7 Application How is data created and used?

6 Presentation How is the data represented to the application?

Is the data in EBCDIC or ASCII format?

5 Session How does an application initiate a connection?

How does an application actually transmit/receive data?

How does an application know data has been received?

4 Transport Should the receiver acknowledge receipt of a packet?

How should the acknowledgement be handled?

Which process should receive the data?

3 Network How is data routed between networks?

2 Data link How do I know when its my turn to transmit?

How do I know which data is for me?

How are collisions handled?

1 Physical What kinds of cabling are supported?

What kinds of connectors are supported?

Whats the longest supported cable segment?


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Media Access Control (MAC) Addresses

Every LAN card has a unique 48-bit MAC address. Every frame of data contains a source and destination MAC.

Hosts accept frames destined for their MAC address.

Hosts ignore frames destined for other MAC addresses.

0x0060B07ef226

These six hex

digits identify

the card

manufacturer

Following

number is

in hex ...

These six hex

digits uniquely

identify this

card

Which frames

are for me?


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Internet Protocol (IP) Addresses

128.1.1.1

Which network

is the host on?

What is the

host's address

on that

network?

Every host on an IP network has a unique, 32-bit IP address. IP addresses make it possible to logically group nodes into IP networks. Network bits within the IP determine which network the host is on.Host bits within the IP distinguish each host from all other hosts on the network. Hosts with identical network bits are said to be on the same IP network.

128.1 Network

128.1.1.1 128.1.1.2


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IP Network Classes

/8 Network

/16 Network

/24 Network

The IP address network/host bit boundary varies from network to network.

Networks with more host bits may have more hosts.

Networks with fewer host bits may have fewer hosts.

8 Network Bits 8 Network Bits 8 Network Bits

8 Network Bits 8 Network Bits

8 Network Bits 8 Host Bits 8 Host Bits 8 Host Bits

8 Host Bits 8 Host Bits

8 Host Bits


7/22 2001 Hewlett-Packard CompanyH3065S C.01 7

The IP Netmask

Netmask 1's identify network bits Netmask 0's identify host bits

111111111 11111111 00000000 00000000Netmask:

255.255.0.0 or0x ff ff 00 00

100000000 00000001 00000001 00000001IP Address:

128.1.1.1/16

Q: How many bits in my IP are network bits?

A: The netmask has the answer!



The IP Network Address

Every host must know which network it is connected to.

Formulate the network address by setting all IP host bits to "0".

128.1.1.1/16128.1.1.2/16

128.1.1.3/16

192.1.1.1/24

192.1.1.2/24

192.1.1.3/24

100000000 00000001 00000000 00000000

Network Address: 128.1.0.0/16

110000000 00000001 00000001 00000000

Network Address: 192.1.1.0/24

Q: Which network am I on?



The IP Broadcast Address

# ping 128.1.255.255

128.1.1.1 128.1.1.2 128.1.1.3

Packets sent

to the network

broadcast addressare received by ALL

hosts on the

network.

Formulate the

broadcast addressby setting all

host bits to "1".



The IP Loopback Address

128.1.1.2128.1.1.1 128.1.1.3

# ping 127.0.0.1

The loopback address, 127.0.0.1, is a special addressthat always references your local host.



Obtaining an IP Address

Private Intranet

Firewall

Obtaining an IP address on

a Private Intranet allows

limited access to the Internet

via a network Firewall.

Obtaining an IP address on

the Public Internet allows

direct connectivity to millions

of hosts worldwide.

Public Internet



IP Address Examples

IP Address

192.66.123.4/24

148.10.12.14/16

9.12.36.1/8

163.128.19.9/16

123.45.65.23/8

199.66.55.4/24

Netmask BroadcastNetwork



Host Names

I can reference nodes

by host name and let

HP-UX automatically

determine the IP

addresses for me!

# telnet oakland

128.1.1.1 sanfran

128.1.1.2 oakland

128.1.1.3 la

128.1.1.4 sandiego

/etc/hosts

Telnet request

To: 128.1.1.2

128.1.1.2 (oakland)



128.1.1.1 080009-000001

128.1.1.2 080009-000002

128.1.1.3 080009-000003

Converting IP Addresses to

MAC AddressesSource MAC: 080009-000001

Destination MAC: 080009-000002

Outbound Frame

128.1.1.2

(oakland)

080009-000002

1. Resolve hostname oakland to an IP address.

2. Lookup the MAC address in the ARP cache corresponding to oakland's IP address.

3. Send the packet to oakland's MAC address.

Example: System sanfran pings system oakland

128.1.1.1 sanfran

128.1.1.2 oakland

128.1.1.3 la

/etc/hosts ARP cache (memory resident)

128.1.1.1

(sanfran)

080009-000001



Populating the ARP Cache

128.1.1.1 080009-000001

128.1.1.2 080009-000002

128.1.1.3 080009-000003

128.1.1.4 incomplete!

ARP cache

$ ping sandiego

128.1.1.4

(sandiego)

128.1.1.3

(la)

128.1.1.2

(oakland)

128.1.1.1

(sanfran)

BroadcastPacket

2

3

4

5

6

128.1.1.4 080009-23EF45

1. sanfran pings sandiego. sanfran resolves sandiego's IP address via /etc/hosts.2. Search for sandiego's IP in the arp cache the IP address is not found in ARP cache.

3. Send ARP broadcast on the local network to find the MAC address for 128.1.1.4.

4. System with the specified IP address responds with a packet containing its MAC.

5. The MAC address and corresponding IP address are added to sanfran's ARP cache.

6. The frame specifically addressed to sandiego's MAC address is sent.

Example: sanfran pings sandiego

1



Putting It All Together

Is the

destination a hostname

or an IP address?

Is the

destination on the

local network?

Look for the destination

IP address in routing table.

Resolve hostname

to corresponding

IP address.

Destination machine responds

with its MAC address.

Record the found MAC address

in the ARP cache for later reference.

Send the packet out on the wire

with the source and destination

MAC and IP addresses.

IP address

hostname

Is the

destination IP address

found in ARP cache?

Yes, on localnetwork

No

Send packet to router

to be forwarded to

destination host.

No Yes

Send a broadcast requesting

the MAC for the destination IP.Use the MAC address found

in ARP cache as the

destination MAC.


17/22 2001 Hewlett-Packard CompanyH3065S C.0117

Managing Packet Flow with TCP

oakland

128.1.1.2

sanfran

128.1.1.1

1

34

5

2

6

1 122 3 3 2

1

2

3

Acknowledgements

1

2

3

Open

Data Packets

Close

Segment

Data

Send

PacketRetransmit

Reassemble

1. Open connection to remote node.

2. Segment data into datagram packets.

3. Send datagrams to destination node.

4. If there is no acknowledgement, retransmit!

5. Close connection after all datagrams are received.

6. Receiver node reassembles datagrams into proper order.

Sending a packet with TCP:



Managing Packet Flow with UDP

Sending a packet with UDP:

128.1.1.2

(oakland)

128.1.1.1

(sanfran)

2

1 3

12

2

1. Packets cannot be segmented or streamed; a packet is always sent as a single message.

2. No connection is opened with the node; the packet is simply sent to the node.3. No acknowledgement is sent back to the original sender.

1 1

Since the original sender never knows if packet is received, sender never retransmits.

The receiver doesnt know if it received all of the intended packets.

With UDP, the applicationis responsible for ensuring data transmission is complete.



Sending Data to Applications via Ports

Problem: Who gets the data?

128.1.1.4

(sandiego)

128.1.1.3

(la)

128.1.1.2

(oakland)

128.1.1.1 (sanfran)

Thousands of packets arrive every minute on the LAN interface card.

How does the network subsystem know to which application to deliver the network packets?

telnetd

$ telnet sanfran $ rlogin sanfran$ ftp sanfran

ftpd rlogind

Network Subsystem

Solution: Assign each application a unique port number.

When each packet is sent, aport numberwill be included in the packet.

The port numbers identify which network application is to receive the packet.

To: port 23 To: port 21 To: port 513

port 23 port 21 port 513



Managing Ports with Sockets

128.1.1.3

(la)

128.1.1.2

(oakland)

128.1.1.1 (sanfran)

telnetd

$ telnet sanfran

$ telnet sanfran

$ telnet sanfran

$ ftp sanfran

Network Subsystem

telnetd

ftpd

telnetd

Problem: Which network application gets the data when multiple instances are present?

Multiple clients can be executing the same network application.

Multiple instances of the network application can be running on the same client.

Solution: Create a unique socket for each process which runs a network application.

A socketis a port number combined with a nodes IP address.A socket connection is the coupling of a client socket address with a server socket address.

To: port 23 To: port 23 To: port 23



More on Socket Connections

128.1.1.2 (oakland)

128.1.1.1 (sanfran)$ telnet sanfran

Network Subsystem

telnetd telnetd

128.1.1.1 . 23

128.1.1.1 . 23

128.1.1.2 . 50001

Socket

Socket

To: port 23 To: port 23

$ telnet sanfran128.1.1.2 . 50002

128.1.1.1.23 128.1.1.1.23

128.1.1.2.50001

telnet telnet

128.1.1.2.50002

Communications between two processes

over the network are uniquely defined by

their socket connection.


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Revisiting the OSI Model7 Application Creates/receives the data.

6 Presentation Determines the format in which to represent the data.

Possible choices are EBCDIC or ASCII format.

5 Session Establishes a unique communication path between client/server.

Sockets are used to communicate between two systems.A socket is an IP address plus a port number.

4 Transport TCP requires that a socket connection be established; UDP does not.TCP requires packets be acknowledged; UDP does not.

TCP is streams-based; UDP is message-based.

3 Network IP addressesdefine a systems network and host number.2 Data link MAC addresses uniquely identify a LAN card.

Ultimately, packets are sent from one MAC address to another.

ARP caches map IP addresses to MAC addresses.

1 Physical The type of media used to connect the machines together.

The type of cabling used for the network.

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