Chapter 8 – Sections & Objectives

• 8.0 Introduction

• 8.1 Subnetting an IPv4 Network

  • Explain how subnetting segments a network to enable better communication.
  • Explain how to calculate IPv4 subnets for a /24 prefix.
  • Explain how to calculate IPv4 subnets for a /16 and /8 prefix.
  • Given a set of requirements for subnetting, implement an IPv4 addressing scheme.
  • Explain how to create a flexible addressing scheme using variable length subnet masking (VLSM).

• 8.2 Addressing Schemes

  • Implement a VLSM addressing scheme.
• 8.3 Design Considerations for IPv6
  • Explain how to implement IPv6 address assignments in a business network.

• 8.4 Summary

8.1 Network Layer Protocols

Network Segmentation

• Broadcast Domains
  • Each router interface connects a broadcast domain.
  • Broadcasts are only propagated within its broadcast domain.
• Problems with Large Broadcast Domains
  • Slow network operations due to the significant amount of broadcast traffic.
  • Slow device operations because a device must accept and process each broadcast packet.
• Reasons for Subnetting
  • Solution: reduce the size of the network to create smaller broadcast domains.
  • Because each broadcast domain connects to a different router interface, each domain needs its own network address space.
  • The process of breaking an address range into smaller address spaces is called subnetting.
  • Network administrators can group devices into subnets that are determined by location, organizational unit or device type.

Subnetting an IPv4 Network

• • Octet Boundaries
    – Subnets can be created based on octet boundaries. (/8, /16 or /24)
  • Subnetting on the Octet Boundary
    – Also known as IPv4 Classes.
    – Uses the octet boundaries to separate network from hosts.
  • Classless Subnetting
    – Uses address bits to separate network from hosts.
    – Allows for much more flexibility.
  • Classless Subnetting Example

• • Creating 2 Subnets
    • A subnet mask of /25 applied to 192.168.10.0, creates two equal subnets, each one with 126 hosts.
  • Subnetting Formulas
    • Use 2n, to calculate the number of subnets.
    • Use 2h-2 to calculate the number of hosts.
    • n is the number allocated to the network portion of the address.
    • h is the number allocated to the host portion of the address.
  • Creating 4 Subnets
    • A subnet mask of /26 applied to 192.168.10.0, creates four equal subnets, each one with 62 hosts.
    • n = 2 and therefore 22 = 4.
    • h = 6 and therefore 26-2 = 62.

Subnetting a /16 and /8 Prefix

• Creating Subnets with a /16 Prefix
  • A subnet mask of /16 applied to 172.16.32.0, creates a network with 65534 hosts.
  • A subnet mask of /18 applied to 172.16.32.0, creates 4 networks with 16382 hosts in each network.
  • A subnet mask of /22 applied to 172.16.32.0, creates 64 networks with 1022 hosts in each network.
• Creating 100 Subnets with a /16 Prefix
  • A subnet mask of /23 applied to 172.16.32.0, creates 128 networks with 510 hosts in each network
• Calculating the Hosts
  • Use 2h-2 to calculate the number of hosts.
  • h is the number allocated to the host portion of the address.
• Creating 1000 Subnets with a /8 Prefix
  • A subnet mask of /18 applied to 20.0.0.0, creates 1024 networks with 16382 hosts in each network

Subnetting to Meet Requirements

• Subnetting Based on Host Requirements
  • Two considerations when planning subnets:
    • The number of host addresses required for each network.
    • The number of individual subnets needed.
• Subnetting Based on Network Requirements
  • Administrators may be asked to subnet an IP range to accommodate a specific number of networks.
  • Think of a company with 7 departments where each department must have its own subnetwork.
  • The number of hosts per subnet, while secondary, is also important.
• Network Requirement Example
  • Assume the range 200.42.98.0/24 was given to the administrator.
  • 7 subnets must be created.
  • Each department will have no more than 29 hosts.
  • A subnet mask of /27 applied to 200.42.98.0/24, creates 8 networks with 30 hosts in each network.

Benefits of Variable Length Subnet Masking

• Traditional Subnetting Wastes Addresses
  • Subnetting based on classes is not very flexible.
  • Results in wasted addresses.
• Variable Length Subnet Masks
  • By varying the mask, an administrator has more control.
  • Less waste.
• Basic VLSM
  • A subnet mask of /30 applied to 200.42.98.0, creates a network with 2 hosts in each network.
  • The network 200.42.98.0/30 would be a perfect match for a serial link.
• VLSM in Practice
  • Consider two routers connected by a Serial link:
  • RouterA would be 200.42.98.1/30 and RouterB would be 200.42.98.2/30.
  • 200.42.98.0/30 is the network address and 200.42.98.3/30 is the broadcast address.

8.2 Addressing Schemes

Structured Design

• Network Address Planning
  • Planning requires decisions on each subnet in terms of size, the number of hosts per subnet and how host addresses will be assigned.
• Planning to Address the Network
  • The Primary Planning Considerations are:
    • Prevent Duplication of Addresses
    • Monitor Security and Performance
    • Provide and Control Access
• Assigning Addresses to Devices
  • Different devices needs may also impact the addressing scheme.
  • Common devices are:
    – End user devices, servers, printers, network devices and gateways

8.3 Design Considerations for IPv6

Structured Design

• The IPv6 Global Unicast Address
  • The IPv6 global unicast address normally consists of a /48 global routing prefix, a 16 bit subnet ID, and a 64 bit interface ID.
• Subnetting Using Subnet ID
  • The subnet ID provides plenty subnets and host support in one subnet.
  • The subnet ID alone allows for creating up to 65,536 /64 subnets.
• IPv6 Subnet Allocation
  • Address waste is not a concern in IPv6.
  • Administrators can concentrate on designing a logical scheme to address the network

8.4 Chapter Summary

Summary

• Implement an IPv4 addressing scheme to enable end-to-end connectivity in a small to medium-sized business network.
• Given a set of requirements, implement a VLSM addressing scheme to provide connectivity to end users in a small to medium-sized network.
• Explain design considerations for implementing IPv6 in a business network.

Section 6.1 New Terms and Commands

• Subnetting
• Classful Boundary
• Classless Subnetting
• Magic number
• Variable Length Subnet Mask (VLSM)
• Global Routing Prefix

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