Version 4.1

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 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 a network identifies an individual host on

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 address  192.168.18.00000000

 all 0’s 192.168.18.0

in the host portion of the 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

Host vs. Network Portion

Network Number Host Number

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

Broadcast Addresses

Network Address

 120.0.0.0

 170.50.0.0.  192.168.10

Broadcast Address

120.255.255.255

170.5.255.255

192.168.10.255

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

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

Available Host Addresses

   The number of available host addresses on a network can be calculated with the formula:

2 ^ host bits – 2

   Network type 255.0.0.0 2 ^ 24 -2 = 255.255.0.0 2 ^ 16- 2 = 255.255.255.0 2 ^ 8 – 2 = Available Hosts 16, 277, 214 65, 534 254 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 1 host portion) st address (all 0’s in the host portion) and the very last address (all 1’s in the

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.

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

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 m

ask 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

organizations.

 addresses are typically assigned to large 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

Class A

  The first bit is always 0 Addresses start with 0 to 126

Class B

  First two bits are always 1 and 0 Addresses start with 128 to 191

Class C

  First three bits are always 1, 1 and 0 Addresses start with 192 to 223

Class D

Class E

1 to 126

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

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.

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

Subnet Mask Formats

Subnet Masks can be written in 2 different formats:

1.



Dotted Decimal format

192.168.1.1 255.255.255.0

2.



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)

4.2: Types of Subnetting

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

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

Example Scenario

Subnet 3 Subnet 2 Subnet 1

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

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 Address:    Network (network bits) Subnetwork (subnet bits) Host (host bits) in a IP

Classful Subnetting

   Traditional

classful subnetting

characteristics: has these 1. Uses a fixed number of subnets 2. Has a fixed number of hosts per subnet 3. All subnets must be the same size 4. 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

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 addresses. wastes a significant number of IP

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

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

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

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

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

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

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

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

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: 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 (2 13 ) An ISP can use the supernet as one large network or divide it into as many smaller networks as needed to meet its requirements.

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.

    

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.

Communicate between Subnets

Subnet 1 Subnet 3 WAN Interfaces LAN Interfaces Subnet 2

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

NAT Advantages

 NAT has several advantages: 1. Saves registered IP addresses  IP addresses can be re-used and many hosts on a single LAN can share globally unique IP addresses 2. Increased security by  Withholds hosts actual IP host addresses from direct Internet access 3. Transparent to end users 4. Adds Scalability to LAN

NAT Disadvantages

1.

2.

3.

Incompatible with certain applications Prevents legitimate remote access to network Requires increased processing by router which negatively affects network performance

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.

NAT at Work

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

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

Inside & Outside Addresses

Inside Global Address = NAT Translated Public IP Address

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

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

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 internal addresses  to represent the different private 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

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

PAT

PAT Security

   PAT conversations use a unique and combination of the private IP address and port number  Example: 192.168.1.106: 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.

IP Nat issues

1.

 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. 2.

 Requires careful network design and equipment selection Routers must support PAT 3.

Requires accurate configuration

 

IPv.6

3 Solutions

were developed to provide to temporarily alleviate the problem of IPv4 address depletion: 1. Subnetting 2. Private IP addressing 3. 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

   

IPv6

Uses a

128 bit

Address Represented as

32 hexadecimal

separated by colons (   8 groups of 4 digits

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

IPv6 Address

IPv6

Improvements

 IPv6 offers many improvement over IPv4: 1. Allows for more address space 2. Creates better space management 3. Allows easier TCP/IP administration 4. Incorporates modern Routing capabilities 5. Provides support for advanced network capabilities

         

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 preserve addresses to provide security and 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