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 are hierarchical
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
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
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 5

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

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. Available Host Addresses
The number of available host addresses on a network can be calculated with the formula: ^ host bits – 2
Network type Available Hosts
^ = , 277, 214
^ = , 534
^ 8 – 2 =
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:
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:
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:
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 19

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, network address, is reserved for loopback testing.
The loopback IP address, is used to test a NIC card to verify that it is sending and receiving signals.

22. Subnet Masks Subnet Masks contain:
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:

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

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 3
Subnet 2
Subnet 1

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 classful subnetting 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 classful subnetting 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: /24
Subnet 3: 2 hosts
/27
Subnet 2: 10 hosts
/27
Subnet 1: 30 hosts
/27

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

34. VLSM
Variable length subnet masking (VLSM) helps solve the limits of classful subnettting
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 classful subnetting, 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 classfull subnetting

36. Example: VLSM Network: 192.168.1.0 /24 Subnet 3: 2 hosts
/30
Subnet 2: 10 hosts
/28
Subnet 1: 30 hosts
/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 = mask
Subnet Address: /27
This provides 30 addresses per subnet
Subnet 2 needs 10 hosts
Take the next available Subnet :
Need 10 hosts, so 4 host bits must be left over
Borrow 4 bits
Subnet mask =
Subnet Address: /28
Subnet 3 needs 2 hosts
Take the next available subnet:
Need 2 hosts, so 2 host bits must be left over
Borrow 6 bits
Subnet mask =
Subnet Address: /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: / 18
/18 bits in the network portion of the address
This block contains the Addresses: to

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
Supernets are created by combining a group of Class C addresses into one large block
This enables addresses to be assigned more efficiently
Example: /19
19 bits are used for the network prefix
This block contains the addresses to
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 Interface must 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
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

51. Inside & Outside Addresses
Inside Global Address = NAT Translated Public IP Address

52. Dynamic NAT
Dynamic NAT dynamically translates each inside local addresses to an inside global address by using 1 public IP address, or a pool of addresses

53. Static NAT
What if one or more of the hosts within a network are running services that need to be accessed from the Internet?
Static NAT translates a permanent registered global address to particular hosts
Static NAT is used for Servers that need a consistent IP address
Static translations ensure that an individual host private IP address is always translated to the same registered global IP address
Static NAT allows hosts on the public network to access selected hosts on a private network

54. PAT
PAT (Port Address Translation) translates multiple inside local addresses to a single global address using Port numbers
PAT is also called NAT overload
PAT translates every inside local address to the same inside global address, by using PORT NUMBERS to represent the different private internal addresses
When a source host sends a message to a destination host, it uses an IP address and port number combination to keep track of each individual conversation with the destination host

55. How PAT works
PAT translates the local source address and port combination in an outgoing packet to a single global IP address and a unique port number above 1024
Each host is translated into the same global IP address, but the port number associated with the conversation is unique.
Responding traffic is addressed to the translated IP address and port number used by the host.
A table in the router contains a list of the inside Local addresses and port numbers

56. PAT

57. PAT Security
PAT conversations use a unique and combination of the private IP address and port number
Example: : 7000
Uses Port numbers above 1024
PAT Maximizes security
Each private IP address/port number translation is ONLY created when a host on the inside network initiates communication
The translation is only in place for the duration of the connection, so a given user does not keep the same global IP address and port number combination after the conversation ends.
Users on the outside network cannot reliably initiate a connection to a host on a network that uses PAT.

58. IP Nat issues
Requires additional network workload to support IP addresses and port translations
Some applications embed an IP address as part of the encapsulated data
The router must replace the source IP addresses and port in the data, and the source addresses in the IP header.
Requires careful network design and equipment selection
Routers must support PAT
Requires accurate configuration

59. IPv.6
3 Solutions were developed to provide to temporarily alleviate the problem of IPv4 address depletion:
Subnetting
Private IP addressing
NAT / PAT
IPv6 was proposed as a permanent solution to the problem of IPv4 address depletion
Outlined in 1998 in RFC 2460
The transition to IPv6 is ongoing

60. IPv6
Uses a 128 bit Address
Represented as 32 hexadecimal digits separated by colons (
8 groups of 4
Ex: 2001:0db8:3c55:0015:0000:0000:abcd:ff13
Uses a 3-part hierarchy:
Global Prefix: assigned to an organization by an Internet names registry
12 Hex digits
Subnet: identifies the Subnet
4 Hex digits
Interface Identifier: identifies the host
16 Hex digits

61. IPv6 Address

62. IPv6 Improvements IPv6 offers many improvement over IPv4:
Allows for more address space
Creates better space management
Allows easier TCP/IP administration
Incorporates modern Routing capabilities
Provides support for advanced network capabilities

63. Summary
Devices that want to communicate over a network need a unique IP address
IP addressing can be tailored to the needs of the network design through the use of custom subnet masks.
A network can be divided into subnets to provide security and preserve addresses
Subnets and custom subnet masks can be created by extending the number of bits used for the network portion of the address
Communication between subnets requires a router
Classful subnetting uses the same subnet mask for each subnet
Classless subnetting gives classful IP addressing schemes more flexibility through the use of variable length subnet masks.
Network Address Translation (NAT) allows a group of private IP addresses to share a small pool of public IP addresses
Port Address Translation (PAT) translates multiple local addresses to a single global IP address, maximizing the use of both private and public IP addresses.
IPv6 offers improvements over IPv4