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Why create a subnet mask?

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. How to Create Subnets. 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0

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Why create a subnet mask?

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  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 to determine which subnet it belongs to • NETWORK/SUBNET/HOST

  2. 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

  3. 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

  4. 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 IP Subnet addressing default subnet masks

  5. 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

  6. How an IP address Is Composed Part of the 32 Bits represents A network ID The remainder is Used to represent A host with the network

  7. 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.

  8. 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 A 10? For class B 110 for class C

  9. How Address Classes Affect a Network Ranges of 1st octet network ID’s A B C

  10. “private” address – cannot be routed on the internet

  11. 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

  12. 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

  13. 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.

  14. 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

  15. Subnetting IP Addresses • Variable length subnet masks • Could subnet a class B into several chunks Network Host Network Subnet Host SubnetMask 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

  16. 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.

  17. 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?

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