Chapter 21 IP Addressing “If we all did the things we are capable of doing, we would literally astound ourselves” - Thomas Alva Edison,

Chapter 2 2 Objectives Recognize and describe the various IP address classes from A to E, and explain how they’re composed and used Describe the IPv4 address limitations, and how techniques like Classless Inter-Domain Routing (CIDR) and use of private IP addresses with Network Address Translation (NAT) ease those limitations Define the terms subnet and supernet, and apply subnetting and supernetting concepts in solving specific network design problems

Chapter 2 3 IP Addressing Basics Different addressing schemes: –Symbolic (eg: –Logical numeric (eg: ) –Physical numeric (eg: 6 byte MAC addresses) Symbolic addresses are easier to remember than a numeric address such as Physical numeric addresses are MAC layer addresses associated with the Data Link layer (of the OSI Reference model) Logical numeric addresses are IP addresses associated with the Network layer

Chapter 2 4 IP Addressing  IPv4 uses 32-bit addresses, commonly represented in dotted decimal notation. Eg: (in binary) (each octet in decimal) Written as: (in dotted decimal notation)  Classful Addresses  Address range is divided into 5 classes (A to E)  Each address has two parts: Network address (Net id) and Host address (Host id) A two-level hierarchy

Chapter 2 5 0Net idHost id (24 bits)Class A 1Net idHost id (16 bits)Class B0 1Net idHost id (8 bits)Class C01 1Multicast group idClass D011 1Reserved for future useClass E0111 Classful Addresses 1

Chapter 2 6 Address ranges for different classes ClassRange A to B to C to D to E to

Chapter 2 7 Classful Addresses Class A - only ~125 networks possible –Each network can support 16,777,214 hosts (2^24 - 2) – is not assigned to a specific network –The address range 10.x.x.x (x: 0-255) is reserved for private network use (as per RFC 1918) –127.x.x.x (x: 0-255) is reserved for loopback testing

Chapter 2 8 Classful Addresses Class B - for moderate to large networks –Each network can support 65,534 hosts (2^16 - 2) –The address range to is reserved for private use Class C - for small networks –Each network can support 254 hosts (2^8 - 2) –The address range is reserved for private use

Chapter 2 9 Types of Addresses Unicast: data sent to a single host (or, an interface on a machine) Broadcast: sent to all hosts on a network –Directed broadcast - host id with all 1’s Eg: A packet sent to is received by all hosts on the network Routers may forward these broadcast packets –Limited broadcast Never forwarded by a router Multicast: sent to a set of hosts that belong to a “multicast” group Host id with all 0’s is not assigned as a host address, but identifies the network.

Chapter 2 10 Subnetting A network can be divided into sub-networks internally, by dividing the host portion of an IP address into a subnet id and a host id within the subnetwork (a three-level hierarchy) This activity of stealing bits from the host portion to further subdivide the network portion of an address is called subnetting a network address, or subnetting

Chapter 2 11 Subnet Mask A 32-bit subnet mask identifies the network and subnet bits in an IP address If a bit value is 1 in the subnet mask, the corresponding bit in the IP address is considered part of the network address

Chapter 2 12 Subnet Masks The simplest form of subnet masking uses a technique called constant-length subnet masking (CLSM), in which each subnet includes the same number of hosts and represents a simple division of the address space made available by subnetting into multiple equal segments Another form of subnet masking uses a technique called variable-length subnet masking (VLSM) and permits a single network address to be subdivided into multiple subnets, in which subnets need not all be the same size

Chapter 2 13 Subnetting Example 1: An large organization is assigned with the network address /16. It needs to support about 150 subnets for different locations. In each subnet, it needs to support about 200 hosts. As the first step, decide the number of bits needed from host bits to represent the subnet ID.

Chapter 2 14 Subnetting Example 1: Subnetting the network by using 8 bits of the 16 host id bits –Subnet mask: –Possible subnets: 2^8 => 256 –Possible hosts per subnet: 2^8 - 2 => 254 –Addresses of subnetworks: (Subnet #0) (Subnet #1) … (Subnet #255)

Chapter 2 15 Subnetting Example 1... For Subnet #0: –A typical host address is x where x = 1 to 254 (eg: ), with a subnet mask of –Also written as: /24 (without having to write the subnet mask) - Binary Count notation –“24” identifies the number of contiguous 1 bits in the subnet mask and is called the “length of the Extended-Network-Prefix” –Directed broadcast addresses of subnet #0:

Chapter 2 16 Subnetting Example 2: An organization is assigned with network address /24. It needs to define 6 subnets for internal departments. The largest subnet need to support 25 hosts. Step 1: Determine the no. of bits needed from the host id bits (8 in this case) to define 6 subnets –3 bits => 8 subnets (2 extra for future expansion) Step 2: Determine whether the remaining host id bits (5 in this case) is sufficient for max. hosts needed per subnet

Chapter 2 17 Subnetting Example 2... Step 2 continued … –5 bits => 2^5 - 2 => 30 hosts per subnet Subnet mask for each subnet: – – Extended network prefix for each subnet: /27 Network addresses: –Base network: /24 –Subnet #0: /27 –Subnet #7: /27

Chapter 2 18 Subnetting Example 2... Valid host addresses for Subnet #2: –Subnet#2: = /27 –Host #1: = /27 –Host #2: = /27 –Host #3: = /27 –…. –Host#16: = /27 –…. –Host#30: = /27 Broadcast address for each subnet: –Host id with all 1’s –For Subnet #2 above: = /27

Chapter 2 19 More Examples... A host IP address is /26. Determine: –the subnet address –directed broadcast address for the subnet –maximum number of possible hosts on the subnet –maximum number of possible subnets (assuming constant length subnet masking)

Chapter 2 20 To find the subnet address... When a host IP address is given, to find the subnet address: –convert the dotted decimal address to binary notation (not necessary to convert decimal digits containing solely network bits to binary) –identify the host bits in the IP address, using the subnet mask or the extended network prefix –set all these host bits to zero –convert the resulting binary number back to dotted decimal notation

Chapter 2 21 To find the subnet address... In /26, there are 26 network bits (26 most significant bits) and 6 (32-26) host bits This means, the decimal digit 110 contains 2 network bits (2 most significant bits) and 6 host bits (6 least significant bits) decimal 110 => binary Host bits are: Setting host bits to 0 => => 64 (decimal) Therefore, subnet address = /26

Chapter 2 22 To find the broadcast address... When a host IP address is given, to find the broadcast address: –convert the dotted decimal address to binary notation (not necessary to convert decimal digits containing solely network bits to binary) –identify the host bits in the IP address, using the subnet mask or the extended network prefix –set all these host bits to 1 –convert the resulting binary number back to dotted decimal notation

Chapter 2 23 To find the broadcast address... As discussed previously, host bits are: Setting host bits to 1 => => 127 (decimal) Therefore, broadcast address = /26

Chapter 2 24 To find the maximum number of possible hosts in a subnet... Number of host bits = 6 (32-26) Max. possible addresses per subnet = 2^6 = 64 As host bits with all 0’s and all 1’s are not valid host addresses, max. number of hosts possible = 64-2 => 62

Chapter 2 25 To find the maximum number of subnets... Number of subnet bits = => 2 (where: 26 = total number of network bits 24 = default network bits in the given Class C address) Max. possible subnets = 2^2 = 4

Chapter 2 26

Chapter 2 27 Variable Length Subnet Masks (VLSM) A limitation of having only a single subnet mask across a given network-prefix is that once the mask is selected, it locks the organization into a fixed number of fixed- sized subnets. In Subnetting Example 1 (subnetting using 8 bits of the host id), there are 256 possible subnets with 254 hosts each. –If a small subnet needs only a max. of 10 hosts, this wastes IP addresses A solution is to allow a subnetted network to use more than one subnet mask (RFC 1009)

Chapter 2 28 VLSM Example: An organization is assigned the network number /16. It plans to divide the address space into 16 equal sized blocks (subnets 0-15), and then to sub-divide subnet #14 into 16 equal-sized blocks. Using 4 bits for subnet id, 16 subnets of the /16 address block are: Base net: = /16 Subnet #0: = /20 Subnet #1: = /20 …. Subnet #14: = /20 Subnet #15: = /20

Chapter 2 29 VLSM Example... Using 4 more bits for sub-subnet id, 16 sub- subnets of Subnet #14 are: Subnet #14: = /20 Subnet #14-0: = /24 Subnet #14-1: = /24 …. Subnet #14-14: = /24 Subnet #14-15: = /24 Host addresses for Subnet #14-1: Host #1: = /24 Host #2: = /24 …. Host #254: = /24 Broadcast address for Subnet #14-1 =

Chapter 2 30 The Vanishing IP Address Space Interim solutions for IPv4 address depletion problem: –IETF introduced a new way to carve up the IP address space—Classless Inter-Domain Routing (CIDR) –RFC 1918 reserves three ranges of IP addresses for private use—a single Class A ( ), 16 Class Bs ( ), AND 256 Class Cs ( ). When used together with Network Address Translation (a.k.a NAT), private IP addresses can help lift the “cap” on public IP addresses

Chapter 2 31 Classless Inter-Domain Routing (CIDR) Abandons the rigid address classes to eliminate the inefficiency in classful addressing CIDR ignores the traditional A, B, and C class designations for IP addresses, and can therefore set the network-host ID boundary wherever it wants to. To use a CIDR address on any network, all routers in the routing domain must “understand” CIDR notation

Chapter 2 32 Classless Inter-Domain Routing (CIDR) Allows more efficient aggregation of routing info –Route Aggregation: Use of a single entry in a routing table to represent address space of several networks –Reduces the size of routing tables in routers Allows Supernetting –Using contiguous blocks of Class C addresses to simulate a single, large address space Documented in RFCs 1517 to 1520 Eg: /20 identifies a network with a 20-bit network prefix

Chapter 2 33 Supernets Supernetting takes the opposite approach to subnetting: by combining contiguous network addresses, it steals bits from the network portion and uses them to create a single, larger contiguous address space for host addresses Example: An organization has the following contiguous Class C addresses / / / /

Chapter 2 34 Supernets The common prefix for all the 4 addresses is: They can be aggregated as: / 22 In the Supernet, the network ID has 22 bits and the host ID has 10 bits The network address of supernet: /22 The broadcast address of supernet: /22 Valid Host addresses: / /22

Chapter 2 35 Summary IP addresses allow identifying individual network interfaces (and therefore computers or other devices as well) on TCP/IP networks With Classful addressing, 5 address classes (A to E) are defined Classes A through C are assigned to individual hosts and consists of network ID and host ID portions

Chapter 2 36 Summary To help ease address scarcity, the IETF created a form of classless addressing called Classless Inter-Domain Routing (CIDR) that permits the network-host boundary basically anywhere Subnetting divides an assigned address space into smaller groups (subnetworks) by using bits from the host portion to form a subnetwork ID

Chapter 2 37 Summary Within the Class A, B, and C IP address ranges, the IETF has reserved private IP address ranges With CIDR, Supernetting is possible. Supernetting allows borrowing bits from the network portion (opposite of subnetting) to be used as host addresses, to form a “Supernet” by combining contiguous Class C addresses

Chapter 2 38 References RFC 1878, Variable Length Subnet Table For IPv4, Dec shtml (on Subnetting Confusion)