Chapter 11: Subnetting IPv4 networks
The chapter introduction connects classful IPv4 addressing’s inflexibility to the need for subnetting and defines subnetting as dividing a larger network into smaller ones.
After this chapter you will be able to
- Classful addressing limitations. Explain why classful IPv4 addressing created a need for subnetting and describe, at a high level, what subnetting does.
- Classful address size problem. Explain why fixed classful network sizes can waste IPv4 addresses when a network needs fewer hosts than its assigned class provides.
- CIDR and subnet meaning. Describe how CIDR permits prefix lengths beyond classful boundaries, and explain what an address block and a subnet are.
- Prefix length size tradeoff. Explain how extending a prefix changes the number of subnets and the number of addresses in each, using the /24 example.
- FLSM and VLSM subnet sizes. Distinguish FLSM from VLSM by describing how each divides an address block into subnets.
- Borrowed bits and subnets. Explain how borrowing host bits extends the network portion and creates subnets within the original address block. Use the /24 examples to show how the borrowed bits determine subnet addresses and why each borrowed bit doubles the number of subnets.
- Five subnet attributes. Describe how to find a subnet’s network address, broadcast address, first and last usable addresses, and maximum host count using its remaining host bits.
- Subnet address from host IP. Apply the section’s binary method to find the network address containing a host IP and prefix length, as with 172.16.20.27/28.
- Unchanged subnetting rules. Explain that subnetting a /16 block uses the same rules as subnetting a /24 block: borrowed bits determine the number of subnets, remaining host bits determine usable host addresses, and an all-zero host portion identifies a subnet’s network address.
- Two-bit /16 subnet example. Describe the result of borrowing two bits from 192.168.0.0/16: four /18 subnets, with 14 host bits and 16,382 usable host addresses per subnet.
- Subnet sizes from a /8 block. Explain that a /8 block has 24 host bits and that borrowing more of them creates more, smaller subnets. Describe the section’s example of borrowing 12 bits from 10.0.0.0/8 to create 4,096 /20 subnets.
- Borrowing across octet boundaries. Explain why borrowing bits across octets does not change the subnetting process, using the section’s 12-bit example.
- Four Subnets From a /23. Apply FLSM to divide 172.25.190.0/23 into four equal subnets, determine their usable host capacity, and identify the first usable address in each.
- Capacity for Two Thousand Subnets. Determine the prefix length and usable host capacity needed when subnetting 10.224.0.0/11 into at least 2,000 equal subnets, and explain why some subnets remain unused.
- Unequal subnet sizes. Explain why VLSM uses subnets of varying sizes instead of equal-sized FLSM subnets in the example /24 block.
- Largest-first subnet assignment. Describe the high-level VLSM process of assigning the largest required subnet first, then placing progressively smaller subnets after it.
- Toronto LAN A subnet size. Explain why Toronto LAN A receives the 10.89.100.0/25 subnet from the 10.89.100.0/24 block.
- Tokyo LAN A network address. Explain why Tokyo LAN A’s network address is 10.89.100.128: it is the first address after Toronto LAN A’s broadcast address, 10.89.100.127.
- Tokyo LAN A subnet size. Explain why 59 hosts require six host bits and a /26 subnet, yielding 62 usable addresses; state the resulting subnet as 10.89.100.128/26.
- Toronto LAN B subnet. Explain why Toronto LAN B is assigned 10.89.100.192/27: its network address follows Tokyo LAN A’s broadcast address, and five host bits provide exactly 30 usable host addresses.
- Tokyo LAN B subnet assignment. Explain why Tokyo LAN B receives 10.89.100.224/28: its network address follows Toronto LAN B’s broadcast address, and four host bits provide enough usable addresses for its 11 hosts.
- WAN subnet prefix choice. Explain why the R1–R2 WAN connection can use either a /30 or a /31 prefix and identify the subnet selected in the example.
- Classful address block assumption. Explain how to interpret a subnetting practice question that gives an IP address and subnet mask but does not state the size of the original address block. Apply the section’s classful assumption to 172.26.0.0.
- Binary focused subnetting practice. Justify practicing subnetting regularly while working through the underlying binary instead of relying on a shortcut such as the magic number method.
What it builds on
- IPv4 address classes (chapter 7) · Introduces IPv4 address classes.
- Attributes of an IPv4 network (chapter 7) · The network and host portions of an IPv4 address
- Binary (chapter 7) · Binary bits and their effect on counts
- The structure of an IPv4 address (chapter 7) · Introduces the network and host portions of an IPv4 address.
Section by section
Introduction
The chapter introduction connects classful IPv4 addressing’s inflexibility to the need for subnetting and defines subnetting as dividing a larger network into smaller ones.
- Classful addressing limitations: Explain why classful IPv4 addressing created a need for subnetting and describe, at a high level, what subnetting does.
Why it matters: This explains why network engineers divide IPv4 networks into subnets. - Classful address prefixes: State how the first bits and prefix lengths distinguish class A, B, and C IPv4 addresses in the classful scheme.
Why it matters: These fixed class prefixes provide the context for why classful addressing was inflexible.
Key terms: classful addressing, subnetting, Class A, Class B, Class C, prefix length
11.1 What is subnetting?
This section explains why classful IPv4 networks waste addresses and how CIDR makes it possible to divide an address block into smaller subnets.
- Classful address size problem: Explain why fixed classful network sizes can waste IPv4 addresses when a network needs fewer hosts than its assigned class provides.
Why it matters: This explains why more flexible IPv4 network sizes are useful. - CIDR and subnet meaning: Describe how CIDR permits prefix lengths beyond classful boundaries, and explain what an address block and a subnet are.
Why it matters: These terms describe the flexibility that makes subnetting possible. - Prefix length size tradeoff: Explain how extending a prefix changes the number of subnets and the number of addresses in each, using the /24 example.
Why it matters: This tradeoff is the basis for choosing subnet sizes.
Key terms: classful addressing, class C network, class B network, usable addresses, Classless Inter-Domain Routing, CIDR, prefix length, address block, subnet, subnetting
11.2 FLSM subnetting
This section introduces FLSM by contrasting its equal-sized subnets with VLSM’s variable-sized subnets and explaining why FLSM is taught first.
- FLSM and VLSM subnet sizes: Distinguish FLSM from VLSM by describing how each divides an address block into subnets.
Why it matters: The size distinction is the foundation for understanding the two approaches to subnetting. - Reasons for using each method: Explain why VLSM is useful in practice and why the section teaches FLSM first.
Why it matters: This explains why the book starts with FLSM even though VLSM uses addresses more efficiently.
Key terms: Fixed-Length Subnet Masking (FLSM), Variable-Length Subnet Masking (VLSM), subnets, VLSM, FLSM, address block
11.2.1 Subnetting /24 address blocks
This section shows how to divide a /24 IPv4 block into subnets, calculate subnet attributes, interpret /24-and-longer masks, and handle /31 point-to-point links.
- Borrowed bits and subnets: Explain how borrowing host bits extends the network portion and creates subnets within the original address block. Use the /24 examples to show how the borrowed bits determine subnet addresses and why each borrowed bit doubles the number of subnets.
Why it matters: Borrowed bits explain both how subnets are formed and how many a given prefix can produce. - Five subnet attributes: Describe how to find a subnet’s network address, broadcast address, first and last usable addresses, and maximum host count using its remaining host bits.
Why it matters: These attributes identify the addresses a host can actually use in a subnet. - Subnet address from host IP: Apply the section’s binary method to find the network address containing a host IP and prefix length, as with 172.16.20.27/28.
Why it matters: This method lets you identify a host’s subnet even when its network address does not end in .0. - Longer prefix subnet masks: Explain what the ones and zeros in a subnet mask mean and why a prefix longer than /24 can produce a mask whose final octet is neither 0 nor 255.
Why it matters: Reading masks correctly helps you recognize subnet boundaries and use the masks required in Cisco IOS configuration. - Point-to-point prefix exception: Distinguish the normal /30 host calculation from the /31 exception for a point-to-point link, and explain why /31 saves addresses.
Why it matters: The /31 exception prevents the ordinary host formula from giving the wrong conclusion for a router-to-router link.
Key terms: host portion, network portion, borrowed bits, prefix length, network address, broadcast address, first usable address, last usable address, maximum number of hosts, dotted decimal
11.2.2 Subnetting /16 address blocks
This section applies the familiar subnetting rules to /16 address blocks, where calculations may extend beyond the final octet.
- Unchanged subnetting rules: Explain that subnetting a /16 block uses the same rules as subnetting a /24 block: borrowed bits determine the number of subnets, remaining host bits determine usable host addresses, and an all-zero host portion identifies a subnet’s network address.
Why it matters: These rules let you carry your /24 subnetting method over to larger address blocks. - Two-bit /16 subnet example: Describe the result of borrowing two bits from 192.168.0.0/16: four /18 subnets, with 14 host bits and 16,382 usable host addresses per subnet.
Why it matters: The example makes the bit-count rules concrete for a /16 block.
Key terms: borrowed bits, host bits, network address, prefix length, 192.168.0.0/16, /18, host addresses
11.2.3 Subnetting /8 address blocks
Explains how a /8 IPv4 block can be divided into subnets, including when borrowed bits cross octet boundaries.
- Subnet sizes from a /8 block: Explain that a /8 block has 24 host bits and that borrowing more of them creates more, smaller subnets. Describe the section’s example of borrowing 12 bits from 10.0.0.0/8 to create 4,096 /20 subnets.
Why it matters: This tradeoff helps you choose a subnet size that fits the number of networks and hosts you need. - Borrowing across octet boundaries: Explain why borrowing bits across octets does not change the subnetting process, using the section’s 12-bit example.
Why it matters: Octet boundaries need not limit where you place a subnet prefix.
Key terms: /8 address block, host bits, borrow, subnets, /20, borrowed bits, octets, 32 bits
11.2.4 FLSM scenarios
Works through two FLSM scenarios to show how borrowed host bits determine equal-sized subnet counts, prefix lengths, and usable host addresses.
- Four Subnets From a /23: Apply FLSM to divide 172.25.190.0/23 into four equal subnets, determine their usable host capacity, and identify the first usable address in each.
Why it matters: This example shows that the same subnetting process works even when the starting block is not /8, /16, or /24. - Capacity for Two Thousand Subnets: Determine the prefix length and usable host capacity needed when subnetting 10.224.0.0/11 into at least 2,000 equal subnets, and explain why some subnets remain unused.
Why it matters: Choosing enough borrowed bits meets the subnet requirement while revealing the host capacity and any spare subnets.
Key terms: FLSM, borrowed bits, prefix length, host bits, first usable address, /22 prefix length, host addresses, extra subnets
11.3 VLSM subnetting
This section explains why VLSM fits differently sized networks into one address block and introduces the largest-first assignment process.
- Unequal subnet sizes: Explain why VLSM uses subnets of varying sizes instead of equal-sized FLSM subnets in the example /24 block.
Why it matters: It explains why having enough addresses in total is not the same as allocating them effectively. - Largest-first subnet assignment: Describe the high-level VLSM process of assigning the largest required subnet first, then placing progressively smaller subnets after it.
Why it matters: Starting with the largest requirement gives the reader a clear way to approach the five allocations. - Router addresses in host counts: State how router IP addresses are treated in the scenario’s host requirements.
Why it matters: Counting router addresses prevents the reader from understating how many addresses a subnet needs.
Key terms: Variable-Length Subnet Masking (VLSM), FLSM, host addresses, VLSM, address block, largest subnet, host, end host, router IP addresses
11.3.1 Assigning Toronto LAN A’s subnet
This section sizes Toronto LAN A’s subnet and identifies the address space left for the remaining VLSM assignments.
- Toronto LAN A subnet size: Explain why Toronto LAN A receives the 10.89.100.0/25 subnet from the 10.89.100.0/24 block.
Why it matters: Choosing the smallest subnet that fits the LAN leaves address space for other networks. - Remaining address space: Describe how much of the original block Toronto LAN A uses and which addresses remain for other subnets.
Why it matters: Tracking the occupied range prevents later subnet assignments from overlapping Toronto LAN A.
Key terms: host bits, host addresses, /24 address block, /25 subnet, address space, 10.89.100.0/25, remaining range of addresses
11.3.2 Assigning Tokyo LAN A’s subnet
The section assigns Tokyo LAN A the next available subnet, sizes it for 59 hosts, and identifies the addresses left in the /24 block.
- Tokyo LAN A network address: Explain why Tokyo LAN A’s network address is 10.89.100.128: it is the first address after Toronto LAN A’s broadcast address, 10.89.100.127.
Why it matters: This identifies where the next subnet begins without overlapping Toronto LAN A. - Tokyo LAN A subnet size: Explain why 59 hosts require six host bits and a /26 subnet, yielding 62 usable addresses; state the resulting subnet as 10.89.100.128/26.
Why it matters: Sizing the subnet to its host requirement determines how much of the address block remains. - Remaining address block range: Describe how the assigned /25 and /26 occupy three-quarters of 10.89.100.0/24, leaving 10.89.100.192 through 10.89.100.255 for the remaining subnets.
Why it matters: Tracking the unassigned range keeps subsequent subnet assignments within the available addresses.
Key terms: broadcast address, network address, host bits, usable addresses, /26 prefix length, 10.89.100.0/24 address block, remaining range of addresses
11.3.3 Assigning Toronto LAN B’s subnet
Assigns Toronto LAN B the next available VLSM subnet, chooses a prefix for 30 hosts, and identifies the addresses left in the /24 block.
- Toronto LAN B subnet: Explain why Toronto LAN B is assigned 10.89.100.192/27: its network address follows Tokyo LAN A’s broadcast address, and five host bits provide exactly 30 usable host addresses.
Why it matters: This shows how to place and size the next subnet without overlapping the previous one. - Subnet sizing for growth: Distinguish the sizing rule for this exercise from real-world planning: use the most efficient prefix with few unused addresses for the scenario, but leave room for future hosts in a real network.
Why it matters: The reason for choosing an exact fit here differs from how you might plan a network expected to grow. - Remaining address range: Describe how the /25, /26, and /27 allocations consume seven-eighths of 10.89.100.0/24, leaving 10.89.100.224 through 10.89.100.255 for the remaining subnets.
Why it matters: Tracking the unassigned range prevents the next subnet from overlapping one already allocated.
Key terms: network address, broadcast address, host bits, /27 prefix length, future growth, most efficient prefix length, 10.89.100.0/24 address block, /25 subnet, /26 subnet, /27 subnet
11.3.4 Assigning Tokyo LAN B’s subnet
Assigns Tokyo LAN B the next available subnet, sizes it for 11 hosts, and identifies the address space left for the WAN connection.
- Tokyo LAN B subnet assignment: Explain why Tokyo LAN B receives 10.89.100.224/28: its network address follows Toronto LAN B’s broadcast address, and four host bits provide enough usable addresses for its 11 hosts.
Why it matters: Choosing both the next available address and a sufficient subnet size avoids overlapping subnets while meeting the LAN’s host requirement. - Remaining address block: State which addresses remain after assigning Tokyo LAN B and why they suffice for the final WAN subnet.
Why it matters: Tracking the unassigned addresses shows that the final WAN connection can still receive a subnet.
Key terms: network address, broadcast address, host bits, /28 prefix length, 10.89.100.0/24 address block, final subnet, WAN connection
11.3.5 Assigning the WAN connection’s subnet
The section assigns a subnet to a two-host WAN link, explains the address savings from VLSM, and practices two subnetting skills tested on the CCNA exam.
- WAN subnet prefix choice: Explain why the R1–R2 WAN connection can use either a /30 or a /31 prefix and identify the subnet selected in the example.
Why it matters: The prefix choice determines how many addresses the two-host link consumes. - Remaining VLSM address space: Describe what remains after assigning the five subnets and why VLSM made that result possible.
Why it matters: It shows the practical benefit of fitting subnet sizes to different address needs. - Prefix and mask matching: Explain how to check a prefix-length-to-subnet-mask pair and identify the correct pairs in the example.
Why it matters: Recognizing the matching mask matters when configuring IP addresses and static routes in Cisco IOS. - Usable host range identification: Describe how to identify a subnet's usable host address range and what simplifies the example exam question.
Why it matters: Finding the range endpoints lets you match a subnet to its usable host addresses.
Key terms: point-to-point connection, /30 prefix length, /31 prefix length, usable addresses, VLSM, FLSM, subnets of varying sizes, prefix length, subnet mask, binary 1s
11.4 Additional subnetting practice
This section guides readers in practicing subnetting, interpreting questions that omit the original address-block size, and focusing on binary understanding rather than shortcuts.
- Classful address block assumption: Explain how to interpret a subnetting practice question that gives an IP address and subnet mask but does not state the size of the original address block. Apply the section’s classful assumption to 172.26.0.0.
Why it matters: This assumption lets you identify the starting block when a practice question leaves it unstated. - Binary focused subnetting practice: Justify practicing subnetting regularly while working through the underlying binary instead of relying on a shortcut such as the magic number method.
Why it matters: Practicing with binary builds understanding that remains useful beyond individual subnetting questions. - Wildcard mask practice questions: Distinguish wildcard-mask questions from the subnetting questions the reader should practice for now.
Why it matters: You need not treat wildcard-mask questions as a gap in your current subnetting practice.
Key terms: classful network, class B address, address block, /16, binary, magic number method, subnetting, wildcard masks, subnet masks
11.5 Summary
Where this is used later
- (chapter intro) (chapter 12) · Introduces subnetting as dividing a network into smaller subnets.
- Layer 3 segmentation with subnets (chapter 12) · The idea of dividing an IP network into subnets.
Concepts explained in this chapter
- subnetworkA subnet is a logical subdivision of an IPv4 network address space with its own network address and prefix length.
- subnettingIPv6 subnetting is the division of an IPv6 address block into smaller subnetworks by allocating bits from the subnet identifier.
- maximum host countThe maximum host count is the number of IPv4 addresses in a network that remain assignable after excluding the network and broadcast addresses.
- netmaskA netmask is a 32-bit value paired with an IPv4 address to identify which address bits belong to the network portion and which belong to the host portion.
- borrowed bitA borrowed bit is a host bit reassigned to the network portion of an IPv4 address during subnetting.
- network addressA network address is the first IPv4 address in a network, with all host bits set to zero, and cannot be assigned to a host.
- prefix lengthA prefix length is the number of leading bits in an IP address that constitute its network portion, written in slash notation such as /24.
- address blockAn address block is a contiguous range of IP addresses that can represent a network or be divided into smaller subnetworks.
- variable-length subnet maskingVariable-length subnet masking is a subnetting method that divides an address block into subnets of different sizes based on address requirements.
- broadcast addressA broadcast address is the last IPv4 address in a network, with every host-portion bit set to one, for delivery to all hosts in that network.
- first usable addressThe first usable address is the first IPv4 address after the network address that can be assigned to a host.
- last usable addressThe last usable address is the IPv4 address immediately before the broadcast address that can be assigned to a host.
- fixed-length subnet maskingFixed-length subnet masking is a subnetting technique that divides an address space into subnets of the same size.
- classless inter-domain routingCIDR notation is slash notation that expresses an IP address's network prefix length.
- host portionA host bit is an IPv4 address bit belonging to the host portion rather than the network portion.
- classful networkingClassful networking is an IPv4 addressing architecture based on the fixed network boundaries and default prefix lengths of address classes A, B, and C.
- class bClass B is an IPv4 address class whose first octet ranges from 128 through 191 and whose default network prefix length is /16.
- ipv4 networkAn IPv4 address block is a contiguous range of IPv4 addresses represented by a common network prefix.
- wildcard maskA wildcard mask is a 32-bit bit mask in which zero bits require corresponding address bits to match and one bits ignore them during address matching.
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