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Network Addressing Structure

Network Addressing Structure. CCNA Discovery2: Chapter 4. Contents. 4.1 : IP Addressing & Subnetting Review 4.2 : VLSM & CIDR 4.3 : NAT and PAT. IP Addresses. IP addresses identify hosts and network devices

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Network Addressing Structure

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  1. Network Addressing Structure CCNA Discovery2: Chapter 4

  2. Contents • 4.1: IP Addressing & Subnetting Review • 4.2: VLSM & CIDR • 4.3: NAT and PAT

  3. IP Addresses • IP addresses identify hosts and network devices • To send and receive messages on an IP network, every host must be assigned a unique 32-bit IP address • IP address are displayed in dotted-decimal notation • 192.168.1.1 • Each of the 4 octets represents 8 bits • IP addresses are hierarchical • The network portion identifies the network that a host belongs to • The host portion identifies an individual host on a network

  4. Network Addresses The network portion of the address, is used to represent the entire network It represents a group of IP addresses that can be used on that network The network address consists of the network field plus all 0’s in the host portion of the address 192.168.18.00000000 192.168.18.0 The Network address is not a usable host IP address Network addresses are only used by routers to decide how to get packets to their destination

  5. Host vs. Network Portion Network Number Host Number

  6. Broadcast Address • A Broadcast Address is the address used to send messages to every host on the same network • A Broadcast Address consists of the Network address, plus all 1’s in the host field • The Broadcast address is NOT a USABLE host address and can not be assigned to a host

  7. Broadcast Addresses Network Address Broadcast Address 120.0.0.0 120.255.255.255 170.50.0.0. 170.5.255.255 192.168.10 192.168.10.255

  8. Usable Host Addresses As we just saw, the Network address and the Broadcast address are NOT usable host addresses A usable host IP address is an IP address that: Is not a Network Address (all 0’s in host field) Is not a Broadcast Address (all 1’s in host field) Is not a reserved Address (127 addresses) Is a Class A, B or C address Only a usable host IP address can be assigned to a host device

  9. Determining Usable Host Addresses Network Usable Hosts Broadcast 10.0.0.0 10.0.0.1 – 10.255.255.254 10.255.255.255 172.16.0.0 172.16.0.1-172.16.255.254 172.16.255.255 192.168.1.0 192.168.1.1-192.168.1.1254 192.168.1.255

  10. Available Host Addresses The number of available host addresses on a network can be calculated with the formula: 2 ^ host bits – 2 Network type Available Hosts 255.0.0.0 2 ^ 24 -2 = 16, 277, 214 255.255.0.0 2 ^ 16- 2 = 65, 534 255.255.255.0 2 ^ 8 – 2 = 254 The reason we always subtract 2 from the total host addresses to determine the available host addresses, is because the network address and broadcast address are NOT usable host address Therefore, every network has 2 addresses that can not be assigned to hosts, the very 1st address (all 0’s in the host portion) and the very last address (all 1’s in the host portion)

  11. IP Address Classes • To create more possible network designations, the 32-bit address space was organized into five classes. • Class A, B, and C: Commercial networks • Class D and E: multicast and experimental • The class of a network is indicated by the values of the first few bits of the IP address, called the high-order bits.

  12. IP Address Classes • Early Networks were only identified with an 8 bit network address • To create more possible network designations, the 32-bit address space was organized into five classes. • Class A, B, and C: Commercial networks • Class D and E: multicast and experimental • Routers needed to be programmed to look beyond the first 8 bits to identify class B and C networks. • Networks were divided in a way that makes it easy for routers and hosts to determine the correct number of network ID bits • The class of a network is indicated by the values of the first few bits of the IP address, called the high-order bits.

  13. Commercial IP Address Classes Class C addresses are usually assigned to small networks Use 3 octets for the network and 1 for the hosts N.N.N.H The default subnet mask is 24 bits: 255.255.255.0 2, 097, 150 (2 ^ 21 – 2) possible networks 254 (2 ^ 8 – 2) available host addresses per network Class B addresses are typically used for medium-sized networks Use 2 octets for the network and 2 for the hosts N.N.H.H The default subnet mask is 16 bits: 255.255.0.0 16, 382 (2 ^ 14 – 2) possible networks 65, 534 (2 ^ 16 – 2) available host addresses per network Class A addresses are typically assigned to large organizations. Use 1 octet for the network and 3 for the hosts N.H.H.H The default subnet mask is 8 bits: 255.0.0.0 126 (2 ^ 7 – 2) possible networks 16, 777, 214 (2 ^ 24 – 2) available host addresses per network

  14. Class A • The first bit is always 0 • Addresses start with 0 to 126

  15. Class B • First two bits are always 1 and 0 • Addresses start with 128 to 191

  16. Class C • First three bits are always 1, 1 and 0 • Addresses start with 192 to 223

  17. Class D

  18. Class E

  19. 1 to 126

  20. Private IP Addresses • Reserved address space for private networks • Private IPs are not routable on the Internet • Many networking devices give out private IPs through DHCP

  21. The Loopback Address There are also private addresses that can be used for the diagnostic testing of devices. This type of private address is known as a loopback address. The class A, 127.0.0.0 network address, is reserved for loopback testing. The loopback IP address, 127.0.0.1 is used to test a NIC card to verify that it is sending and receiving signals.

  22. Subnet Masks A subnet mask is a 32 bit address which tells devices which part of the IP address is network and which part is host Let routers & hosts figure out which network or subnet an IP address belongs to Subnet Masks contain: all 1’s in the network field all 0’s in the host field Example Subnet Masks: 255.255.255.0 255.255.0.0 255.255.255.128 255.254.0.0

  23. Subnet Mask Formats Subnet Masks can be written in 2 different formats: • Dotted Decimal format • 192.168.1.1 255.255.255.0 • Bit-Mask Format • 192.168.1.1 /24 • This indicates that there are 24 bits ( 24 1’s) in the network and subnetwork portion of the address (255.255.255.0)

  24. 4.2: Types of Subnetting

  25. The Need for Subnetting • Networks continued to grow and connect to the Internet throughout the 80s and 90s, with many organizations adding hundreds, and thousands of hosts to their network. • This created 3 needs or problems: • The need to create separate LANS within a company for security or management purposes. • Increased hosts increased the broadcast traffic which decreased network performance • There are a limited number of Class B and C addresses available

  26. Example Scenario • An ISP customer has outgrown its initial network installation - the original integrated wireless router is overloaded with traffic from both wired and wireless users • They have a Class C network address • Solution: • Add a 2nd networking device (larger integrated service router) • When adding a device, it is a good practice to place the wired and wireless users on separate local subnetworks to increase security • The new network configuration requires that the existing Class C network be divided into at least three subnetworks

  27. Example Scenario Subnet 1 Subnet 3 Subnet 2

  28. Subnets Defined • RFC 917 defines Internet Subnets • The Subnet mask is the method routers use to isolate the network portion from an IP address. • Routers read subnet masks left to right, bit for bit • Bits set to 1 are read as part of the network ID • Bits set to 0 are read as part of the host ID

  29. Altering the Address Hierarchy • In the original IP address hierarchy, there are 2 levels: • Network field (network bits) • Host field (host bits) • Subdividing a classful network adds a new level to the network hierarchy • It creates 3 levels of Hierarchy in a IP Address: • Network (network bits) • Subnetwork (subnet bits) • Host (host bits)

  30. Classful Subnetting • Traditional classfulsubnetting has these characteristics: • Uses a fixed number of subnets • Has a fixed number of hosts per subnet • All subnets must be the same size • Each subnet must use the same subnet mask • Also known as fixed-length subnetting • All subnets must be the same size, which means that the maximum number of hosts that each subnet can support is the same for all subnets created • The more bits that are taken for the subnet ID, the fewer bits left for host IDs

  31. Limits of Classful Subnetting • The original classfulsubnetting method required that all subnets of a single classed network be the same size. • This was because routers did not include subnet mask information in their routing updates • A router programmed with 1 subnet address and mask on an interface automatically applied that same mask to the other network subnets in its routing table. • This limitation required the use of fixed-length subnets and subnet masks • This technique wastes a significant number of IP addresses.

  32. Example: Classful Subnetting Network: 192.168.1.0 /24 Subnet 3: 2 hosts 172.16.1.96 /27 Subnet 2: 10 hosts 172.16.1.64 /27 Subnet 1: 30 hosts 192.168.1.32 /27

  33. Example: Classful Subnetting • Original Network address: 192.168.1.0 /24 • Subnet 1 needs 30 hosts so subnets will have to be created that support at least 30 hosts • 3 bits are borrowed = 255.255.255.224 mask • 5 host bits are left unborrowed • This provides 30 addresses per subnet • Subnet Addresses are: • 192.168.1.32 • 192.168.1.64 • 192.168.1.96 • This wastes many addresses in Subnet 2 and 3

  34. VLSM • Variable length subnet masking (VLSM) helps solve the limits of classfulsubnettting • VLSM allows an address space to be divided into subnets of various sizes • This is done by subnetting subnets • Characteristics of VLSM • Each subnet can be a different size • Each subnet can be designed to support the number of hosts needed • Each subnet can have a different subnet mask

  35. How does it work • In order for VLSM to work, Routers must be aware of how the network was subnetted. • With classfulsubnetting, we know that the Subnet Mask information was not shared with other routers • With VLSM, routers must share subnet mask information, so routers will know how many bits have been used for the network portion of each subnet address • VLSM saves thousands of IP addresses that would be wasted with traditional classfullsubnetting

  36. Example: VLSM Network: 192.168.1.0 /24 Subnet 3: 2 hosts 192.168.1.80 /30 Subnet 2: 10 hosts 192.168.1.64 /28 Subnet 1: 30 hosts 192.168.1.32 /27

  37. Example: VLSM • Original Network Address: 192.168.1.0 /24 • Subnet 1 needs 30 hosts: • Need 30 hosts, so 5 bits must be left in the host portion • Borrow 3 bits = 255.255.255.224 mask • Subnet Address: 192.168.1.32 /27 • This provides 30 addresses per subnet • Subnet 2 needs 10 hosts • Take the next available Subnet : 192.168.1.64 • Need 10 hosts, so 4 host bits must be left over • Borrow 4 bits • Subnet mask = 255.255.255.240 • Subnet Address: 192.168.1.64 /28 • Subnet 3 needs 2 hosts • Take the next available subnet: 192.168.1.80 • Need 2 hosts, so 2 host bits must be left over • Borrow 6 bits • Subnet mask = 255.255.255.252 • Subnet Address: 192.168.1.80 /30

  38. CIDR • CIDR = Classless Inter-Domain Routing • CIDR is a type of network addressing that ignores the traditional network classes (Class A, B and C) • CIDR Assigns Blocks of Addresses, based on the number of hosts needed • Can be though of as assigning a Subnet of a Class A or Class B address to a company as a block of Addresses • It identifies networks based solely on the number of bits in the network prefix • Example: 172.16.64.0 / 18 • /18 bits in the network portion of the address • This block contains the Addresses: 172.16.64.1 to 172.16.127.255

  39. CIDR • CIDR protocols freed routers from using only the high-order bits to determine the network prefix • registered IP addresses do NOT need to be assigned by class • Before CIDR, an ISP requiring 3,000 host addresses could request either a full Class B address space or multiple Class C network addresses to meet its requirements. • With a Class B address space, the ISP would waste thousands of registered addresses. • With multiple Class C addresses, it could be difficult to design the ISP network so that no single section required more than 254 host addresses. • By ignoring the traditional address classes, CIDR enables ISPs to request a block of addresses based on the number of host addresses it requires. • CIDR is defined in RFC 1519

  40. Supernets • Supernetsare created by combining a group of Class C addresses into one large block • This enables addresses to be assigned more efficiently • Example: 192.168.0.0/19 • 19 bits are used for the network prefix • This block contains the addresses 192.168.32.1 to 192.168.63.255 • This allows 8,190 possible host addresses (213) • An ISP can use the supernet as one large network or divide it into as many smaller networks as needed to meet its requirements.

  41. Why learn classed addressing? • Although classed addressing and fixed-length subnet masking are becoming less common, it is important to understand how these addressing methods work. • Many networking devices still use the default subnet mask if no custom subnet mask is specified.

  42. Router Interface Addressing • Each subnet is a separate network and a Router is needed to communicate between Subnets • Every Router Interfacemust have a valid host IP Address: this includes both WAN and LAN interfaces • WAN Interfaces: when 2 routers are connected, there must be a separate network, or subnet assigned to the connection between them • The interfaces on both routers must be assigned host IP addresses in that network or subnet • LAN Interfaces: Each router interface connected to a LAN must have an IP address in the same subnet as the LAN • Each router interface is the default gateway for its subnet • Usually, router interfaces are assigned either the first or last host address available in the subnet. This assures consistency.

  43. Communicate between Subnets Subnet 3 WAN Interfaces LAN Interfaces Subnet 1 Subnet 2

  44. 4.3: NAT • Network Address Translation • NAT allows a group of private users to access the Internet by sharing one or more public IP addresses • NAT translates private IP addresses into 1 or more public IP addresses for routing on the Internet

  45. NAT Advantages • NAT has several advantages: • Saves registered IP addresses • IP addresses can be re-used and many hosts on a single LAN can share globally unique IP addresses • Increased security by • Withholds hosts actual IP host addresses from direct Internet access • Transparent to end users • Adds Scalability to LAN

  46. NAT Disadvantages • Incompatible with certain applications • Prevents legitimate remote access to network • Requires increased processing by router which negatively affects network performance

  47. NAT Analogy • As a company adds employees, at some point, they no longer run a public phone line directly to each employee desk. • Instead, they use a system that allows the company to assign each employee an extension number. • The company can do this because not all employees use the phone at the same time. • Using private extension numbers enables the company to purchase a smaller number of external phone lines from the phone company.

  48. NAT at Work

  49. Inside vs. Outside Network • Inside local network • A network that is part of the privately addressed LAN • Outside global network • A network that is external to the LAN and does not recognize the private addresses assigned to hosts on the LAN

  50. Inside & Outside Addresses • Inside local address • A Private IP address configured on a host on an inside network • Must be translated before it can travel outside the local network addressing structure • Inside global address • The NAT translated IP address • The IP address of an inside host as it appears to the outside network • Outside local address • The Destination address of the packet while it is on the local network • Usually, this is the same as the outside global address. • Outside global address • The Public IP address of an external host

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