Chapter 8: Subnetting IP Networks

Chapter 8: Subnetting IP Networks

1. Broadcast Domains

   • Other devices
     • A device uses Address Resolution Protocol (ARP) which sends Layer 2 broadcasts to a known IPv4 address on the local network to discover the associated MAC address.
   • Services
     • A host typically acquires its IPv4 address configuration using the Dynamic Host Configuration Protocol (DHCP) which sends broadcasts on the local network to locate a DHCP server.

2. Problems with Large Broadcast Domains

   • Slow network operations due to the significant amount of traffic it can cause
   • Slow device operations because a device must accept and process each broadcast packet

3. Octet Boundaries

   • Every interface on a router is connected to a network. The IPv4 address and subnet mask configured on the router interface are used to identify the specific broadcast domain.

4. Classless Subnetting

   • The examples seen so far borrowed host bits from the common /8, /16, and /24 network prefixes. However, subnets can borrow bits from any host bit position to create other masks.
     • /25 row - Borrowing 1 bit from the fourth octet creates 2 subnets supporting 126 hosts each.
     • /26 row - Borrowing 2 bits creates 4 subnets supporting 62 hosts each.
     • /27 row – Borrowing 3 bits creates 8 subnets supporting 30 hosts each.
     • /28 row – Borrowing 4 bits creates 16 subnets supporting 14 hosts each.
     • /29 row – Borrowing 5 bits creates 32 subnets supporting 6 hosts each.
     • /30 row – Borrowing 6 bits creates 64 subnets supporting 2 hosts each.

5. Creating 2 Subnets

   • IPv4 Network address
     • is 192.168.1.0 and contains all 0 bits in the host portion of the address.
   • First IPv4 host address
     • is 192.168.1.1 and contains all 0 bits plus a right-most 1 bit in the host portion of the address.
   • Last IPv4 host address
     • is 192.168.1.126 and contains all 1 bits plus a right-most 0 bit in the host portion of the address.
   • IPv4 Broadcast address
     • is 192.168.1.127 and contains all 1 bits in the host portion of the address.

6. Creating 100 Subnets with a /16 Network

   • Consider a large enterprise that requires at least 100 subnets and has chosen the private address 172.16.0.0/16 as its internal network address.

7. Calculating the Hosts

   • To calculate the number of hosts each subnet can support, examine the third and fourth octet. After borrowing 7 bits for the subnet, there is one host bit remaining in the third octet and 8 host bits remaining in the fourth octet for a total of 9 bits that were not borrowed.

8. Creating 1000 Subnets with a /8 Network

   • Some organizations, such as small service providers or large enterprises, may need even more subnets. Take, for example, a small ISP that requires 1000 subnets for its clients.

9. Subnetting Based on Host Requirements

   • There are two considerations when planning subnets
     • the number of host addresses required for each network
     • the number of individual subnets needed

10. Subnetting Based on Network Requirements

    • Sometimes a certain number of subnets is required, with less emphasis on the number of host addresses per subnet.

11. Traditional Subnetting Wastes Addresses

    • Using traditional subnetting, the same number of addresses is allocated for each subnet.

12. VLSM in Practice

    • Using the VLSM subnets, the LAN and WAN segments can be addressed without unnecessary waste.
      • Default gateway for Building A hosts (192.168.20.0/27) will be 192.168.20.1.
      • Default gateway for Building B hosts (192.168.20.32/27) will be 192.168.20.33.
      • Default gateway for Building C hosts (192.168.20.64/27) will be 192.168.20.65.
      • Default gateway for Building D hosts (192.168.20.96/27) will be 192.168.20.97.

13. Assigning Addresses to Devices

    • End user clients
    • Servers and peripherals
    • Servers that are accessible from the Internet
    • Intermediary devices
    • Gateway

14. The IPv6 Global Unicast Address

    • IPv6 subnetting requires a different approach than IPv4 subnetting. The same reasons for subnetting IPv4 address space in order to manage network traffic also apply to IPv6.

15. Subnetting Using the Subnet ID

    • The 16 bit subnet ID section of the IPv6 global unicast address can be used by an organization to create internal subnets.
      • Create up to 65,536 /64 subnets. This does not include the possibility of borrowing any bits from the interface ID of the address.
      • Support up to 18 quintillion host IPv6 addresses per subnet (i.e., 18,000,000,000,000,000,000).

16. IPv6 Subnet Allocation

    • With over 65,000 subnets to choose from, the task of the network administrator becomes one of designing a logical scheme to address the network.