Chabot College
ELEC 99.05
Addressing & Routing Foundation Concepts
CISCO NETWORKING ACADEMY
Switches & Bridges (review)
• Layer-2 device
• Reduce large collision domains
• Uses MAC address table to
determine whether or not to forward
a frame
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Switches & Bridges (review)
• Make “low level” decisions
• Flood the network if address
unknown
• Always forwards broadcasts
– Excess broadcast traffic can result
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Layer 2 Switch Limitations
Switches & Bridges:
• Don’t handle device moves well
• Can’t control broadcast traffic
• Lack path determination abilities
• Have limited address capacity
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Layer 2 Switch Limitations
• Switch doesn’t handle device
moves well.
– When a device is moved from one
switch port to another, the switch’s
MAC address table will be wrong until
it is “flushed” or “ages out”.
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Layer 2 Switch Limitations
• Switch can’t control broadcast
traffic.
– Switches (and bridges) always forward
broadcast traffic, which is essential to
LAN operation.
– Broadcasts only stop at the edge of the
network.
– Excess broadcast traffic can result.
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Layer 2 Switch Limitations
• Path determination abilities
– Almost no ability to determine paths when
there are multiple routes from source to
destination.
– No ability to change paths when a route
becomes unavailable.
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Layer 2 Switch Limitations
• Limited address capacity
– There are 10’s of millions of MAC addresses
in the world, but the typical switch can store
only a few thousand MACs.
– Fine for LANs, but inadequate for
internetworks (networks of networks).
– This is the key limitation!
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Layer 3 solves these problems!
• Allows device moves through
logical addresses.
• Uses routers to control broadcast
propagation.
• Provides path determination
through routing.
• Uses hierarchical addresses
to support world-wide
addressing.
CISCO NETWORKING ACADEMY
3
Logical Order
In order to impose logical order on
a network, a different kind of
address needs to be used.
Network Layer
Logical Address (Software)
Data Link Layer
Physical Address (Hardware)
Physical Layer
No address
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Physical v. Logical Address
• Physical Address
– Layer 2
– aka MAC address, hardware address,
Ethernet address
– burned in card
– can’t be changed without changing card
– flat scheme (like Social Security
Numbers)
– Analogy: your name
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Physical v. Logical Address
• Logical Address
– Layer 3
– aka protocol address
– set by administrator in software
– can be easily changed
– hierarchical scheme like Phone Numbers
(area code, prefix) or ZIP codes
– Analogy: your mailing address
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Layer 3 addresses
• Logical or Layer 3 addresses come
in different flavors, depending on
the layer-3 protocol used:
– TCP/IP = IP address
– Novell IPX = IPX address
– AppleTalk = AT address
etc.
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Addresses at Layers
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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TCP/IP or IPX/SPX or
AppleTalk
Logical address
MAC address
Ethernet
Logical Address Composition
Though the exact length and format of
a logical (layer 3) address differs
depending on the protocol, all logical
addresses share this basic formula:
NETWORK
NUMBER
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HOST NUMBER
Logical Addresses Explained
Phone Numbers:
510-723-7464 = Alameda County
510-723-7464 = Hayward Exchange
510-723-7464 = Particular Hayward Phone
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Logical Addresses Examples
• AppleTalk address
400:22
network 400, host 22
• IPX (Novell)
4b39.00c0.4f31.03d2
network 4b39, host 00c0.4f31.03d2
• IP (Internet)
207.81.104.15
network 207.81.104, host 15
CISCO NETWORKING ACADEMY
Logical Addresses Explained
• AppleTalk address
400:22
network 400, host 22
• IPX (Novell)
4b39.00c0.4f31.03d2
network 4b39, host 00c0.4f31.03d2
• IP (Internet)
207.81.104.15
network 207.81.104, host 15
CISCO NETWORKING ACADEMY
Logical Addresses Explained
• AppleTalk address
400:22
network 400, host 22
• IPX (Novell)
4b39.00c0.4f31.03d2
network 4b39, host 00c0.4f31.03d2
• IP (Internet)
207.81.104.15
network 207.81.104, host 15
CISCO NETWORKING ACADEMY
Routers
• Routers:
– internetworking devices that work at Layer 3
– understand and use logical addresses
– keep tables of information about networks
– can make path determinations
– don’t normally forward broadcasts
– can support multiple layer 3 protocols
(TCP/IP, IPX, AppleTalk) at the same time
CISCO NETWORKING ACADEMY
Routers at work
Don’t know
Z, but I know
net #7.
NETWORK 1
NETWORK 2
R
A
R
NETWORK 4
NETWORK 5
NETWORK 3
Node 1.A is sending to Node 7.Z
NETWORK 6
R
NETWORK 7
R
Z
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The IP Address
IP addresses are made up of 32 bits:
10101001110001110100010110001001
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Binary IP Addresses
In the binary numbering system,
each digit has two possible values:
0 or 1
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Binary IP Addresses
Each value is a binary digit, or bit
for short.
01111000
Eight bits together make a unit
called a byte. In IP addresses, bytes
are called octets (group of eight).
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Binary IP Addresses
An octet that is all zeros has a
decimal value of 0:
Binary = 00000000
Decimal = 0
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Binary IP Addresses
An octet that is all ones has a
decimal value of 255:
Binary = 11111111
Decimal = 255
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Binary IP Addresses
In decimal, each place value is a
power of ten.
103
102
101
100
1000
100
10
1
2
3
4
2
We read the number 2342 as twothousand three-hundred forty-two.
CISCO NETWORKING ACADEMY
Binary IP Addresses
In binary, each place value is a power of
two.
27
26
25
24
23
22
21
20
128
64
32
16
8
4
2
1
1
1
0
0
1
1
1
1
The octet 11001111 is equivalent to 207
in decimal.
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Binary IP Addresses
The 32 bits of an IP address are
grouped into 4 bytes:
10101001110001110100010110001001
10101001 11000111 01000101 10001001
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IP Addresses
We use dotted notation to represent
the value of each byte (octet) of the IP
address in decimal.
10101001 11000111 01000101 10001001
169
199
69
137
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IP Addresses
An IP address has two parts:
– network number
– host number
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IP Addresses
• Which bits refer to the network
number?
• Which bits refer to the host
number?
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IP Addresses
• The answers to these questions
vary.
• To find out for a specific address,
you need to know how to convert
decimal numbers to binary numbers
(and back again).
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