Why create a subnet mask? Tells device which part of an address is the network # including the...
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Transcript of Why create a subnet mask? Tells device which part of an address is the network # including the...
![Page 1: Why create a subnet mask? Tells device which part of an address is the network # including the subnet and which part is the host Filters the node IP address.](https://reader035.fdocuments.net/reader035/viewer/2022062518/56649de05503460f94ad9a48/html5/thumbnails/1.jpg)
![Page 2: Why create a subnet mask? Tells device which part of an address is the network # including the subnet and which part is the host Filters the node IP address.](https://reader035.fdocuments.net/reader035/viewer/2022062518/56649de05503460f94ad9a48/html5/thumbnails/2.jpg)
Why create a subnet mask?
• Tells device which part of an address is the network # including the subnet and which part is the host
• Filters the node IP address to determine which subnet it belongs to
• NETWORK/SUBNET/HOST
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How to Create Subnets 27 26 25 24 23 22 21 20
128 64 32 16 8 4 2 1
1 0 0 0 0 0 0 0 128
1 1 0 0 0 0 0 0 192
1 1 1 0 0 0 0 0 224
1 1 1 1 0 0 0 0 240
1 1 1 1 1 0 0 0 248
1 1 1 1 1 1 0 0 252
1 1 1 1 1 1 1 0 254
1 1 1 1 1 1 1 1 255
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The class determines which part of the address belongs to the
network and which part belongs to the host
Class A
NNNNNNNN.hhhhhhhh.hhhhhhhh.hhhhhhhh
Class B
NNNNNNNN.NNNNNNNN.hhhhhhhh.hhhhhhhh
Class C
NNNNNNNN.NNNNNNNN.NNNNNNNN.hhhhhhhh
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IP Subnet addressing default subnet masks
In Binary Form
Class A
11111111.00000000.00000000.00000000
Class B
11111111.11111111.00000000.00000000
Class C
11111111.11111111.11111111.00000000
• Class A -
255.0.0.0
• Class B - 255.255.0.0
• Class C - 255.255.255.0
![Page 6: Why create a subnet mask? Tells device which part of an address is the network # including the subnet and which part is the host Filters the node IP address.](https://reader035.fdocuments.net/reader035/viewer/2022062518/56649de05503460f94ad9a48/html5/thumbnails/6.jpg)
Restrictions on subnets
• Network addresses of all 0’s in the host portion are reserved for specifying the network
• Network addresses of all 1’s in the host portions are reserved for the broadcast address
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How an IP address Is Composed
Part of the 32Bits representsA networkID
The remainder isUsed to
representA host with the
network
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Network & Host IDs• Network ID – Each network has a unique
network number– Each Network connected to the Internet has to
have a globally unique ID – no other Internet-connected network in the world can have the same Network ID
• Host ID – – Within a given network – Host IDs are used to identify
hosts • Hosts – any device that needs to be addressed by an IP address
- computers, printers, routers, etc.
– Host IDs must be unique within a given network.
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How Bits Are Set Up for Each IP Address Class
Note – This shows the binary values in the first 3 bits of the 3 classes:0?? For class A10? For class B110 for class C
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How Address Classes Affect a Network
0 0 0 0 0 0 0 1
1 0 0 0 0 0 0 0
1 1 0 0 0 0 0 0
0 1 1 1 1 1 1 0
1 0 1 1 1 1 1 1
1 1 0 1 1 1 1 1
Ranges of 1st octet network ID’s
A
BC
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“private” address – cannot be routed on the internet
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IP Address Guidelines• First Octet Network ID Rules
– Network ID cannot be 127.– ID bits cannot be all “1”s.– ID bits cannot be all “0”s.
• For class B or C Network IDs– Second octet (and the third octet for class C
networks) can be any number from 1 -255, or 00000000 to 11111111
• Class B: 131.0.x.x or 131.255.x.x are OK• Class C: 200.0.0.x or 200.255.255.x or
200.255.0 are all OK
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IP Address Guidelines• First Octet Network ID Rules
– Network ID cannot be 127.– ID bits cannot be all “1”s.– ID bits cannot be all “0”s.
• For class B or C Network IDs– Second octet (and the third octet for class C
networks) can be any number from 1 -255, or 00000000 to 11111111
• Class B: 131.0.x.x or 131.255.x.x are OK• Class C: 200.0.0.x or 200.255.255.x or
200.255.0 are all OK
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Problems with default masks
• They only provide for a single network segment– Class C – 255.255.255.0 allows for a maximum of 254 hosts on the
segment– Class B – 255.255.0.0 allows for a maximum of 65,534 hosts on the
segment– Class C – 255.0.0.0 allows for a maximum of 16,777,214 hosts on
the segment– Beyond class C networks, current LAN technologies will not support
anywhere near the maximum number of hosts/segment
• Since there is only 1 network segment:– Difficult to use different topologies in the LAN (Ethernet, FDDI,
Token Ring)– Difficult to have a geographically dispersed LAN connected using a
WAN technology.
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Common masks• Masks
– 255.255.252.0 /22 1024 hosts– 255.255.254.0 /23 512 hosts
– 255.255.255.0 0 /24 256 hosts– 255.255.255.128 0 + 128 /25 128 hosts– 255.255.255.192 128 + 64 /26 64 hosts– 255.255.255.224 192 + 32 /27 32 hosts– 255.255.255.240 224 + 16 /28 16 hosts– 255.255.255.248 240 + 8 /29 8 hosts255.255.255.252 248 + 4 /30 4 hosts
– 255.255.255.254 252 + 2 /31 not usable– 255.255.255.255 254 + 1 /32 single host– Learn or memorize them.– Or, use the addition trick in column 2
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Subnetting IP Addresses
• Variable length subnet masks– Could subnet a class B into several chunks
Network Host
Network HostSubnet
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0SubnetMask
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Problem #1: Lifetime of Address Space
• Example: an organization needs 500 addresses. A single class C address not enough (256 hosts). Instead a class B address is allocated. (~64K hosts) That’s overkill -a huge waste.
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IPv4 AddressingDotted Decimal Notation
• Dotted Decimal Notation– Four bytes (8 bits = 1 byte) per address– Each byte separated by a dot– Each byte expressed in decimal notation
• Example: – Dotted Decimal Notation: 192.16.224.254– Binary Notation?:
– What is the minimum decimal value any byte can be assigned?
– What is the maximum decimal value any byte can be assigned?