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Chapter 5: Addressing (Part 2 of 3)

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Chapter 5: Addressing (Part 2 of 3) Agenda Special Addresses Classful - Subnetting Classful - Supernetting – PowerPoint PPT presentation

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Title: Chapter 5: Addressing (Part 2 of 3)


1
  • Chapter 5 Addressing (Part 2 of 3)
  • Agenda
  • Special Addresses
  • Classful - Subnetting
  • Classful - Supernetting

2
Multihomed devices
  • As we mentioned, any device with one or more
    connections to the Internet will need an IP
    address for EACH connection such devices are
    called multihomed devices.
  • A Router could be a multihomed device

3
Example of direct broadcast address
Router sending to all hosts on a network
If the hostid is all 1s, its called a
broadcast address and the router use it to send
a packet to all host in a specific network. In
this case, hosts 20, 64, 126 and etc. will
receive the packet from the router
Example of limited broadcast address
Host sending to all other hosts on a network
If the hostid and netid are all 1s, its called
a limited broadcast address. If the host wants
to send a packet to all host in a specific
network, it would use this address. The router
would block this address so that data stays
contained within a specific network.
4
Example of this host on this address
IP-less Host sending message to bootstrap server
An address of all 0s is used during bootstrap
time if the host doesnt know its IP address.
The un-named host sends an all 0 source address
and limited broadcast (all 1s) destination
address to the bootstrap server.
Example of specific host on this network
Host sending to some other specific host on a
network
An address with a netid of all 0s is used by a
host or router to send another host with in the
same network a message.
5
Example of loopback address
  • The IP address with the 1st byte equal to 127 is
    used for the loop back address.
  • Loopback address is used to test software on a
    machine the packet never leaves the machine
    it returns to the protocol software it checks
    your NIC and TCP/IP software
  • Example a ping command can send a packet with
    a loopback address as the destination address to
    see if the IP software is capable of receiving
    and processing a packet.

6
Chapter 5
Subnetting
7
SUBNETTING
  • When we talked about CLASSFUL addressing we
    realized the problem of wasted host addresses and
    depleting available network addresses.
  • Why wasted addresses ? Because there is a single
    owner of the entire block block cant be
    shared with other owners
  • In subnetting, a network is divided into several
    smaller autonomous or self-contained networks
    called subnetworks or subnets each subnet will
    have its own set of addresses
  • Typically, there are 2 steps in reaching a
    destination first we must reach the network
    (netid) and then we reach the destination
    (hostid)
  • With subnets, there could be atleast 3 steps, (1)
    netid, (2) subnet id, and (3) hostid

8
A network with two levels of hierarchy (not
subnetted)
The 2 level approach is not enough some times
you can only have 1 physical network in
example, all host are at the same level no
grouping
9
A network with three levels of hierarchy
(subnetted)
(0-63)
(64-127)
With subnetting, hosts can be grouped
(128-191)
(192-255)
10
Addresses in a network with and without subnetting
With subnetting, there are 3 levels (versus 2
levels). Partition the hostid space into subnetid
and hostid. (1st) network, (2nd) subnetwork and
(3rd) host
11
Similar to Hierarchy concept in a telephone number
12
Default mask and subnet mask
13
Finding the Subnet Address Given an IP address,
we can find the subnet address the same way we
found the network address in the previous
chapter. We apply the mask to the address. We can
do this in two ways straight or short-cut.
Straight Method In the straight method, we use
binary notation for both the address and the mask
and then apply the AND operation to find the
subnet address.
Short-Cut Method If the byte in the mask is
255, copy the byte in the address. If the byte
in the mask is 0, replace the byte in the address
with 0. If the byte in the mask is neither 255
nor 0, we write the mask and the address in
binary and apply the AND operation.
14
Subnet Mask Form
  • In the early days, non-contiguous 1s masks were
    used (0s and 1s could alternate)
  • Today, as a best practice, contiguous 1s masks
    are used
  • In either case, the black box can perform the
    masking process

15
Example 1
What is the subnetwork address if the destination
address is 200.45.34.56 and the subnet mask is
255.255.240.0?
Solution 11001000 00101101 00100010
00111000 11111111 11111111 11110000
00000000 11001000 00101101 00100000
00000000 The subnetwork address is 200.45.32.0.
16
Recall - 5-bit Address Space Illustration
1-bit Netid case (no subnets) 16
addresses/block Number of blocks 2 Address range
per block 0 to 15 Netids 0, 1 Network
Addresses 00000, 10000 Broadcast Addresses
01111, 11111
17
5-bit Address Space Illustration
subnet
1-bit Subnet case Number of blocks/networks
2 Number subnets per block 2 8
addresses/subnet Address range per subnet 0 to
7 Subnet ids 0, 1 Network Addresses 00000,
01000, 10000, 11000 Broadcast Addresses 00111,
01111, 10111, 11111
18
5-bit Address Space Illustration
subnet
2-bit Subnet case Number of blocks/networks
2 Number subnets per block 4 4
addresses/subnet Address range per subnet 0 to
3 Subnet ids 00, 01, 10, 11 Network Addresses
00000, 00100, 01000, 01100 10000, 10100,
11000, 11100 Broadcast Addresses 00011,
00111, 01011, 01111 10011, 10111, 11011,
11111
19
Illustrating the mask concept (1 of 3)
netid
What is the mask ? 10000 If address 11101 is
masked, what is the result ?
result
address
1 0 0 0 0
20
Illustrating the mask concept (2 of 3)
subnet
What is the mask (subnet mask) ? 11000 If
address 11101 is masked, what is the result ?
result
address
1 1 0 0 0
21
Illustrating the mask concept (3 of 3)
subnet
What is the mask (subnet mask) ? 11100 If
address 11101 is masked, what is the result ?
result
address
1 1 1 0 0
22
Example 2
What is the subnetwork address if the destination
address is 19.30.84.5 and the mask is
255.255.192.0?
23
Comparison of a default mask and a subnet mask
A portion of the hostid space is divided between
some contiguous 1s and 0s
24
The number of subnets must be a power of 2.
Determine the number of subnets added by looking
at the number of 1s added to the default mask and
performing 2 raised to that number
For example, 23 8 subnets
25
Example 3
A company is granted the site address 201.70.64.0
(class C). The company needs six subnets. Design
the subnets.
Solution
The number of 1s in the default mask is 24
(class C).
The company needs six subnets. This number 6 is
not a power of 2. The next number that is a power
of 2 is 8 (23). We need 3 more 1s in the subnet
mask. The total number of 1s in the subnet mask
is 27 (24 3). The total number of 0s is 5 (32 -
27). The mask would be
26
Solution (Continued)
11111111 11111111 11111111 11100000 or
255.255.255.224 The number of subnets is 8. The
number of addresses in each subnet is 25 (5 is
the number of 0s) or 32.
27
Example 3
28
Example 4
A company is granted the site address 181.56.0.0
(class B). The company needs 1000 subnets. Design
the subnets.
Solution
The number of 1s in the default mask is 16 (class
B).
The company needs 1000 subnets. This number is
not a power of 2. The next number that is a power
of 2 is 1024 (210). We need 10 more 1s in the
subnet mask. The total number of 1s in the subnet
mask is 26 (16 10). The total number of 0s is 6
(32 - 26).
The mask is 11111111 11111111 11111111
11000000 or
255.255.255.192. The number of
subnets is 1024. The number of addresses in each
subnet is 26 (6 is the number of 0s) or 64.
29
Example 4
Subtract 63 from 255 to get 192
30
SUPERNETTING
  • Although class A and B addresses are dwindling
    there are plenty of class C addresses
  • The problem with C addresses is, they only have
    256 hostids not enough for any midsize to large
    size organization especially if you plan to
    give every computer, printer, scanner, etc.
    multiple IP addresses
  • Supernetting allows an organization the ability
    to combine several class C blocks in creating a
    larger range of addresses
  • Note breaking up a network subnetting
  • Note combining Class-C networks supernetting

31
Assigning or Choosing Class C Blocks
  • When assigning class C blocks, there are two
    approaches (1) random and (2) superblock
  • Random Approach the routers will see each block
    as a separate network and therefore, for each
    block there would be an entry in the routing
    table a router contains an entry for each
    destination network
  • Superblock Approach instead of multiple routing
    table entries, there would be a single entry.
    However, the choices of blocks need to follow a
    set of rules
  • 1 the of blocks must be a power of 2 (ie. 1,
    2, 4, 8 )
  • 2 blocks must be contiguous (no gaps between
    blocks)
  • 3 the 3rd byte of the first address in the
    superblock must be evenly divisible by the number
    of blocks ie. if the of blocks is N, the 3rd
    byte must be divisible by N

32
Example 5
A company needs 600 addresses. Which of the
following set of class C blocks can be used to
form a supernet for this company? 198.47.32.0
198.47.33.0 198.47.34.0 198.47.32.0 198.47.42.0
198.47.52.0 198.47.62.0 198.47.31.0 198.47.32.0
198.47.33.0 198.47.34.0 198.47.32.0
198.47.33.0 198.47.34.0 198.47.35.0
Solution 1 No, there are only three blocks. Must
be a power of 2 2 No, the blocks are not
contiguous. 3 No, 31 in the first block is not
divisible by 4. 4 Yes, all three requirements
are fulfilled. (1. Power of 2, 2. Contiguous and
3. 3rd byte of 1st address is divisible by 4
32/48)
33
Example 8
A supernet has a first address of 205.16.32.0 and
a supernet mask of 255.255.248.0. How many blocks
are in this supernet and what is the range of
addresses?
Solution
  • The default mask has 24 1s because 205.16.32.0 is
    a class C.
  • Because the supernet mask is 255.255.248.0, the
    supernet has 21 1s.
  • Since the difference between the default and
    supernet masks is 3, there are 23 or 8 blocks in
    this supernet.
  • Because the blocks start with 205.16.32.0 and
    must be contiguous, the blocks are 205.16.32.0,
    205.16.33.0, 205.16.34.0. 205.16.39.0.
  • The first address is 205.16.32.0. The last
    address is 205.16.39.255.
  • The total number of addresses is 8 x 256 2048

34
Explain Supernetting Conceptually
Back out this bit from netid into host id
Causes these 2 blocks to combine as a single block
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