IP Addressing, IPv6, and Core Network Services

Introduction

Internet Protocol (IP) addressing is the logical foundation of routed communication. Every device that sends or receives an IP packet must be identified within an addressing structure that supports local delivery, routing, growth, security, and operational administration. In a small network, addresses may be configured manually. In an enterprise, cloud, campus, or service-provider environment, addressing must be designed systematically and supported by services such as Dynamic Host Configuration Protocol (DHCP), Domain Name System (DNS), Internet Control Message Protocol (ICMP), Neighbor Discovery Protocol (NDP), and network address translation (NAT).

This chapter develops an integrated approach to IPv4 and IPv6 addressing. It explains subnetting, variable-length subnet masking (VLSM), classless inter-domain routing (CIDR), route summarization, IPv6 address formation, Stateless Address Autoconfiguration (SLAAC), DHCPv6, NDP, and address-management practices. The emphasis is not only on calculating addresses but also on making defensible design decisions.

Learning Objectives

By the end of this chapter, you should be able to:

• calculate IPv4 network, broadcast, host, and subnet ranges;
• design addressing plans using subnetting, VLSM, and CIDR;
• summarize routes without creating incorrect or overlapping aggregates;
• explain IPv6 address structure, prefixes, interface identifiers, and address types;
• compare SLAAC, stateful DHCPv6, and stateless DHCPv6;
• describe the roles of NDP and ICMPv6;
• explain DHCP, DNS, NAT, and IP address management (IPAM);
• analyze an organizational addressing requirement and produce a practical dual-stack plan;
• identify operational, security, governance, and transition concerns.

1. The Network-Layer Addressing Model

An IP address identifies a logical interface, not necessarily a physical device. A router uses the network prefix to determine where a packet should be forwarded and the interface-specific portion to identify the destination within that network. IPv4 addresses contain 32 bits; IPv6 addresses contain 128 bits.

A prefix length specifies how many leading bits identify the network. For example, 192.0.2.0/24 has 24 network bits and 8 host bits. The notation is classless: the prefix length, rather than the old Class A, B, or C categories, determines the size of the network.

A sound addressing plan separates at least the following concerns:

• Location: site, building, floor, region, or cloud zone;
• Function: users, servers, voice, management, guest access, or infrastructure;
• Routing boundary: where a subnet begins and ends;
• Security boundary: where filtering and policy enforcement occur;
• Growth: reserved capacity for additional hosts, sites, and services.

An address plan should be documented before devices are configured. It should identify the prefix owner, allocation date, purpose, gateway, DNS servers, DHCP scope, VLAN, routing domain, and security classification.

2. IPv4 Addressing and Subnetting

2.1 IPv4 address structure

An IPv4 address is written as four decimal octets, each representing 8 bits. A prefix divides the address into a network portion and a host portion. If a prefix has length n, then the number of host bits is h = 32 - n. Under the usual subnetting model, the number of addresses in a subnet is:

where A is the total number of addresses and n is the prefix length. For an ordinary IPv4 subnet, the network address and directed-broadcast address are reserved, so usable host addresses are:

This subtraction applies to conventional subnets with at least two host bits. Special cases exist: a /31 can support a point-to-point link under RFC 3021, and a /32 identifies one host or route rather than a conventional multi-host subnet.

The network address has all host bits set to zero. The broadcast address has all host bits set to one. For 192.0.2.64/26, there are 6 host bits, 64 total addresses, network address 192.0.2.64, and broadcast address 192.0.2.127. Usable hosts range from 192.0.2.65 through 192.0.2.126.

2.2 Fixed-length subnetting

Fixed-length subnetting divides a larger network into equal-sized subnets. Suppose an organization owns 198.51.100.0/24 and needs four equal networks. Borrowing 2 host bits produces a /26 prefix because 24 + 2 = 26. The number of subnets is:

where b is the number of borrowed bits. Thus, S = 2² = 4. Each /26 contains 64 addresses and 62 ordinary usable hosts:

• 198.51.100.0/26, hosts .1–.62, broadcast .63;
• 198.51.100.64/26, hosts .65–.126, broadcast .127;
• 198.51.100.128/26, hosts .129–.190, broadcast .191;
• 198.51.100.192/26, hosts .193–.254, broadcast .255.

Fixed-length subnetting is easy to administer but may waste addresses when departments have different sizes.

2.3 VLSM

Variable-length subnet masking assigns different prefix lengths to different requirements. The correct design procedure is to sort requirements from largest to smallest, allocate the largest suitable block first, and ensure that every allocation begins on a valid boundary.

Worked example. An organization has 10.20.0.0/24 and requires networks for 100, 50, 20, and 10 hosts.

1. 100 hosts require at least 102 usable addresses, so 7 host bits are needed: /25 gives 126 usable addresses.
2. 50 hosts require 62 usable addresses: /26.
3. 20 hosts require 30 usable addresses: /27.
4. 10 hosts require 14 usable addresses: /28.

A valid allocation is:

Addresses .240–.255 remain available for future use. VLSM improves utilization, but it increases documentation and routing complexity. It is therefore most effective when combined with hierarchical summarization.

2.4 CIDR and route summarization

CIDR permits arbitrary prefix lengths and supports hierarchical routing. Route summarization advertises one aggregate route instead of multiple more-specific routes. For example, four contiguous /24 networks—172.16.0.0/24, 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24—can be summarized as 172.16.0.0/22. The four networks are contiguous, and the first network begins on a valid /22 boundary.

A safe summary must satisfy three conditions:

1. the component networks are contiguous;
2. their count is a power of two for a single exact aggregate;
3. the first network is aligned to the aggregate boundary.

Summarization reduces routing-table size and limits the propagation of internal changes. However, an overly broad summary can attract traffic for destinations that do not exist. A summary should therefore be advertised only when the summarizing router can correctly handle all addresses within the advertised range, often by using a discard or null route for unused portions.

Longest-prefix matching resolves overlapping routes: a router selects the route with the greatest number of matching leading bits. This permits a broad summary and a more-specific exception to coexist.

2.5 Private addressing and NAT

RFC 1918 defines private IPv4 ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. These addresses are not globally routed on the public Internet. NAT translates addresses between private and public realms. Port address translation (PAT), also called NAT overload, maps many internal address-and-port pairs to one public address using different transport-layer ports.

NAT conserves public IPv4 addresses and can simplify renumbering, but it is not a complete security mechanism. It can complicate end-to-end communication, logging, troubleshooting, peer-to-peer applications, and some protocols that embed IP addresses in payloads. IPv6 generally restores globally unique addressing, although firewalls and privacy controls remain necessary.

3. IPv6 Addressing

3.1 Representation and hierarchy

IPv6 addresses contain 128 bits and are written as eight groups of four hexadecimal digits. Leading zeroes may be omitted within a group, and one consecutive sequence of zero groups may be compressed with :: exactly once. For example, 2001:0db8:0000:0000:0000:0000:0000:0042 becomes 2001:db8::42.

A typical global unicast design uses a provider or organizational prefix, a subnet identifier, and an interface identifier. Many enterprise designs allocate a /48 per site and /64 per LAN, although actual allocations depend on the address authority and design policy. A /64 contains 64 interface-identifier bits, which supports SLAAC and avoids the need to conserve individual host addresses.

IPv6 commonly uses:

• Global unicast: globally routable addresses, normally beginning within a provider- or registry-assigned range;
• Unique local address (ULA): private organizational addressing from fc00::/7, commonly using locally generated /48 prefixes;
• Link-local: fe80::/10, required on IPv6-enabled interfaces and valid only on the local link;
• Multicast: ff00::/8, used for one-to-many communication;
• Loopback: ::1;
• Unspecified: ::, used when a host has no address yet.

IPv6 has no broadcast address. Multicast groups replace broadcast functions, reducing unnecessary delivery to hosts that do not need a message.

3.2 Interface identifiers and privacy

An interface identifier may be generated from a hardware-derived method, randomly, or through stable-but-private algorithms. Traditional modified EUI-64 construction can expose a relationship to a hardware address; modern operating systems often use temporary privacy addresses for outbound client traffic. Stable addresses may still be used for servers and infrastructure, but their generation and logging should be controlled.

A /64 is not normally subdivided into smaller LAN prefixes merely to conserve addresses. IPv6 design assumes abundant address space and values consistent prefix lengths, route aggregation, and autoconfiguration.

4. SLAAC, DHCPv6, and NDP

4.1 Neighbor Discovery Protocol

NDP is an ICMPv6-based protocol that performs functions formerly associated with IPv4 ARP and adds router discovery and address-related operations. Important messages include:

• Router Solicitation (RS): a host asks for router information;
• Router Advertisement (RA): a router announces prefixes, default-router information, and configuration flags;
• Neighbor Solicitation (NS): a host resolves a neighbor’s link-layer address or checks reachability;
• Neighbor Advertisement (NA): a response or unsolicited update about a neighbor;
• Redirect: a router informs a host of a better first-hop router.

NDP uses multicast rather than broadcast. Neighbor Unreachability Detection (NUD) checks whether a neighbor remains reachable. Duplicate Address Detection (DAD) uses NS messages to determine whether an address is already in use before assignment.

Because fraudulent RAs can redirect traffic, enterprises may deploy RA Guard, switch protections, authenticated control-plane mechanisms, segmentation, and monitoring. RA Guard must be implemented carefully because evasion and fragmented-message issues have affected some deployments.

4.2 SLAAC

With SLAAC, a host receives an RA containing a prefix and forms its own interface address. It performs DAD and installs a default route learned from the router. SLAAC is efficient for large user populations because a central server need not assign every address.

The RA includes flags that communicate configuration policy. The autonomous (A) flag indicates that hosts may use the advertised prefix for SLAAC. The managed (M) and other-configuration (O) flags indicate whether DHCPv6 should provide addresses or additional information. Exact operating-system behavior can vary, so network engineers should test the intended combination.

4.3 DHCPv6

Stateful DHCPv6 assigns addresses and maintains leases in a central service, providing stronger inventory control. Stateless DHCPv6 provides additional parameters such as DNS information while hosts obtain their addresses through SLAAC. DHCPv6 does not replace NDP: routers and default gateways are still learned through RAs.

A common misconception is that DHCPv6 alone supplies the default gateway. In standard IPv6 operation, the default-router information comes from the RA.

5. ICMP and ICMPv6

ICMP communicates network-layer errors, diagnostics, and control information. IPv4 ICMP supports echo request and reply, destination unreachable, time exceeded, and redirects. Tools such as ping and traceroute depend on these functions, although filtering can make results incomplete.

ICMPv6 is more fundamental because NDP, router discovery, DAD, and path maximum-transmission-unit discovery depend on it. Blocking all ICMPv6 is therefore likely to break IPv6 rather than improve security. Firewalls should permit necessary types according to a documented policy while limiting unsolicited or untrusted traffic.

Path MTU discovery is especially important. A sender should learn the largest packet size that can traverse a path without fragmentation. In IPv6, routers do not fragment transit packets; the source must adjust packet size after receiving appropriate ICMPv6 feedback. Incorrect filtering can create a “black hole” in which small packets work but larger application traffic fails.

6. Core Network Services

6.1 DHCP for IPv4

DHCP automates IPv4 configuration. The familiar DORA sequence is Discover, Offer, Request, and Acknowledge. A DHCP scope defines an address pool and options such as subnet mask, default gateway, DNS servers, lease duration, and domain name.

A DHCP relay forwards client broadcasts across routed boundaries to a server. The relay adds information identifying the receiving interface or VLAN, allowing the server to select the correct scope. Reservations associate a client identifier or hardware address with a predictable address, but reservations should not be confused with static configuration: the address is still administered by DHCP.

Lease duration is a policy decision. Short leases reclaim addresses quickly but increase renewal traffic and dependence on service availability. Long leases reduce transaction volume but may retain unused addresses.

6.2 DNS

DNS maps names to resource records. Common records include A for IPv4, AAAA for IPv6, CNAME for aliases, MX for mail exchange, NS for authoritative name servers, PTR for reverse mapping, and TXT for text-based policy or verification data.

A recursive resolver obtains answers on behalf of clients and may cache them according to time to live (TTL). An authoritative server publishes data for a zone. DNS design should include redundant authoritative servers, controlled zone transfers, secure administrative access, split-horizon or split-DNS policy where internal and external answers differ, and monitoring of expiry and resolution failures.

DNS is not merely a convenience. Applications, identity systems, service discovery, cloud platforms, and security controls frequently depend on it. DNS Security Extensions (DNSSEC) provide origin authentication and integrity for signed DNS data, but they do not encrypt ordinary DNS queries. Encrypted transports such as DNS over TLS or DNS over HTTPS address confidentiality in different deployment models.

6.3 IP address management

IPAM is the disciplined administration of address space, prefixes, assignments, DNS relationships, DHCP scopes, ownership, and lifecycle. A useful IPAM record includes:

• prefix and prefix length;
• site, VLAN, and business owner;
• gateway and routing context;
• DHCP and DNS associations;
• allocation status and utilization;
• approval, change, and retirement history;
• security classification and contact information.

IPAM may be a specialized platform or an integrated database connected to network automation. It should be treated as a source of operational truth, while live device state and discovery data are used to detect drift. Access should follow least privilege, and changes should be auditable.

7. Address-Plan Design Method

A professional design normally follows these steps:

1. Gather requirements. Identify sites, segments, host counts, growth rates, point-to-point links, cloud networks, regulatory boundaries, and high-availability needs.
2. Select private or provider-assigned space. Avoid overlap with merger partners, suppliers, VPN users, and common cloud defaults.
3. Create a hierarchy. Allocate a parent block per region or site, then child blocks per function or security zone.
4. Choose subnet sizes. Use VLSM for IPv4 and normally /64 LANs in IPv6.
5. Reserve growth. Do not consume every available block merely because it exists.
6. Plan routing and summarization. Ensure site allocations can be represented by aggregates without overlap.
7. Define services. Document DHCP scopes, relay locations, DNS servers, NTP, gateways, and failover.
8. Define security policy. Identify permitted management, client, server, guest, and inter-zone flows.
9. Validate. Check boundaries, duplicate ranges, route advertisements, failover behavior, and address utilization.
10. Operate and revise. Update IPAM, monitor usage, and manage changes through review.

8. Illustrative Case: Dual-Stack University Campus

Background

This illustrative case concerns Northbridge University, which has one main campus, a data center, guest Wi-Fi, laboratories, voice services, and a research network. It has received 10.40.0.0/16 for internal IPv4 use and a global IPv6 /48, 2001:db8:1200::/48 for documentation purposes in this example. The campus expects 1,200 student devices, 350 staff devices, 180 voice endpoints, 100 servers, and several small infrastructure networks.

Problem

The existing flat network has excessive broadcast traffic, inconsistent gateway addresses, overlapping VPN ranges, and no reliable record of assignments. The university wants segmentation, dual-stack operation, room for growth, and summarized routes from buildings to the campus core.

Analysis and approach

The design team first allocates a /20 IPv4 block to the main campus: 10.40.0.0/20. Building blocks are aligned on /22 boundaries:

• Administration: 10.40.0.0/22;
• Teaching buildings: 10.40.4.0/22;
• Research: 10.40.8.0/22;
• Data center and infrastructure: 10.40.12.0/22.

Each /22 can be summarized toward the core. Within the teaching block, VLSM assigns a /21-equivalent capacity across multiple VLANs only if the original allocation is redesigned at a suitable boundary; therefore, the engineers do not arbitrarily claim a /21 summary for noncontiguous VLANs. Instead, they allocate valid child prefixes—for example, /23 for a large student VLAN, /24 for staff, /25 for voice, and /26 for management—while preserving contiguous building-level ranges.

For IPv6, each major site receives a /52 from the /48, and each VLAN receives a /64. For example, the administration site may use 2001:db8:1200:1000::/52 conceptually as a site allocation, with individual /64s assigned to VLANs. The exact nibble-aligned hierarchy is documented so that routing summaries remain possible.

The campus deploys:

• IPv4 DHCP scopes per VLAN with redundant DHCP servers;
• IPv6 RAs on every IPv6 LAN;
• SLAAC for general clients plus stateless DHCPv6 for DNS and domain information;
• stateful DHCPv6 for selected managed segments requiring lease records;
• internal DNS zones with redundant resolvers and reverse zones;
• NAT/PAT for IPv4 Internet access;
• globally routed IPv6 with firewall policy rather than relying on NAT;
• RA Guard and DHCP snooping where supported;
• IPAM integrated with change management.

Outcome

The campus obtains predictable routing boundaries, summarized building advertisements, controlled address allocation, and a migration path from IPv4-only applications to dual-stack services. Guest users are isolated from internal networks, voice receives a dedicated policy and address pool, and the data center can apply service-specific firewall rules.

Learning points

The case demonstrates that subnet calculations alone do not produce a good design. Address hierarchy, route aggregation, DNS and DHCP resilience, IPv6 control-plane protection, non-overlapping VPN space, and accurate IPAM are equally important. It also shows why a summary must be based on real binary boundaries rather than a visually convenient decimal range.

9. Challenges, Security, and Governance

Address exhaustion, overlapping private ranges, undocumented exceptions, stale DNS records, rogue DHCP servers, fraudulent IPv6 router advertisements, and inaccurate IPAM data are common operational risks. Controls should include authenticated administration, DHCP snooping where appropriate, RA protections, secure DNS change processes, logging, configuration backups, and periodic reconciliation between IPAM and device state.

Privacy also matters. Address records can reveal users, locations, device identities, and activity patterns. Organizations should define retention periods, access controls, and lawful monitoring practices. IPv6 temporary addresses can reduce long-term tracking for client-originated traffic, but logs must still support legitimate security and operational requirements.

Automation and AI-assisted operations can identify unused addresses, detect duplicate assignments, compare intended and actual configurations, and predict capacity needs. Such systems should not be permitted to make unreviewed routing or address changes. Training data, generated recommendations, false positives, explainability, and approval controls must be considered. Automation is valuable only when its source data and governance are reliable.

10. Best Practices

• Use hierarchical, non-overlapping allocations with documented ownership.
• Allocate IPv4 with VLSM and preserve aligned blocks for summarization.
• Use consistent IPv6 /64 LAN prefixes and meaningful site or function boundaries.
• Treat ICMPv6 and required NDP traffic as essential control traffic.
• Provide redundant DHCP and DNS services and test relay and failover behavior.
• Separate guest, user, voice, server, and management networks.
• Avoid assuming NAT is a firewall or that IPv6 does not require filtering.
• Record every allocation and change in IPAM.
• Monitor utilization, duplicate addresses, rogue services, DNS health, and route growth.
• Test dual-stack applications, especially DNS, MTU discovery, authentication, and logging.
• Reserve space for mergers, VPNs, cloud connectivity, and future sites.

Chapter Summary

IP addressing combines binary structure, routing logic, service operation, and governance. IPv4 subnetting uses prefix lengths to divide networks into addressable units; VLSM improves utilization by assigning different sizes, while CIDR and route summarization support scalable routing. The network and broadcast addresses are normally excluded from IPv4 host use, subject to special cases such as /31 point-to-point links.

IPv6 expands the address space to 128 bits, replaces broadcast with multicast, and relies on ICMPv6-based NDP for router discovery, neighbor resolution, DAD, and reachability. SLAAC provides scalable host configuration, while DHCPv6 can supply stateful address assignment or additional configuration. RAs remain central to default-router discovery.

DHCP, DNS, NAT, and IPAM form the operational foundation around addressing. A professional plan is hierarchical, non-overlapping, secure, documented, summarized where possible, and designed for growth. Effective engineers evaluate both numerical correctness and practical consequences such as resilience, privacy, security, and maintainability.

Key Terms

IPv4: A 32-bit Internet Protocol addressing system.

IPv6: A 128-bit Internet Protocol addressing system.

Prefix length: The number of leading bits identifying a network.

Subnet mask: An IPv4 mask distinguishing network and host bits.

VLSM: Variable-length subnet masking; assigning different subnet sizes to different requirements.

CIDR: Classless Inter-Domain Routing; prefix-based addressing and routing.

Route summarization: Advertising one aggregate route for several more-specific networks.

Longest-prefix match: Selecting the route with the greatest number of matching leading bits.

NAT: Network address translation between address realms.

PAT: Port address translation, allowing many flows to share a public IPv4 address.

SLAAC: Stateless Address Autoconfiguration for IPv6 hosts.

DHCPv6: IPv6 configuration service supporting stateful and stateless operation.

NDP: Neighbor Discovery Protocol, an ICMPv6-based IPv6 control protocol.

Router Advertisement: An ICMPv6 message carrying IPv6 prefix and router information.

DAD: Duplicate Address Detection.

ICMP: Internet Control Message Protocol for errors, diagnostics, and control.

DNS: Domain Name System for name and service resolution.

DHCP: Dynamic Host Configuration Protocol.

IPAM: Management of address allocation, prefixes, leases, and related records.

RA Guard: A network protection that restricts unauthorized IPv6 router advertisements.

Review Questions

Short Answer Questions

1. What is the difference between a network address and a broadcast address in a conventional IPv4 subnet?
2. Why does VLSM generally use the largest host requirement first?
3. What information does an IPv6 Router Advertisement provide?
4. Why is ICMPv6 more operationally essential to IPv6 than ICMP is to basic IPv4 forwarding?
5. Distinguish stateful DHCPv6 from stateless DHCPv6.

Long Answer Questions

1. Explain how prefix length, host bits, total addresses, and usable IPv4 hosts are related. Include the assumptions behind the usual usable-host formula.
2. Compare fixed-length subnetting, VLSM, CIDR, and route summarization in terms of efficiency, scalability, and administrative complexity.
3. Describe the operation of NDP, including RS, RA, NS, NA, DAD, and NUD.
4. Explain how DHCP, DNS, NAT, and IPAM interact in an enterprise network.
5. Discuss the security and governance implications of dual-stack IPv4/IPv6 deployment.

Analytical / Application Questions

1. An organization owns 192.0.2.0/24 and needs subnets for 90, 40, 20, and 10 hosts. Design a VLSM allocation, show each prefix and range, and identify remaining capacity.
2. Four contiguous /24 networks are proposed for summarization, but the first network begins at a decimal value that is not aligned to the proposed aggregate boundary. Analyze whether the summary is valid and identify the operational risk of using it anyway.
3. Design a dual-stack addressing and service plan for a branch office with user, voice, server, guest, and management VLANs. Justify your IPv4 subnet sizes, IPv6 prefix strategy, SLAAC/DHCPv6 choices, DNS design, security controls, and IPAM records.

Multiple Choice Questions with Answers

Question 1

How many usable host addresses are available in an ordinary IPv4 /26 subnet?

A. 30

B. 62

C. 64

D. 126

Correct Answer: B. 62

Question 2

Which allocation best fits a requirement for 50 ordinary IPv4 hosts?

A. /28

B. /27

C. /26

D. /25

Correct Answer: C. /26

Question 3

Which condition is necessary for a single exact route summary?

A. The component networks must be randomly distributed

B. The component networks must be contiguous and properly aligned

C. All component networks must use different protocols

D. The summary must always be a /24

Correct Answer: B. The component networks must be contiguous and properly aligned

Question 4

Which IPv6 mechanism provides default-router information to hosts?

A. DHCPv6 only

B. ARP

C. Router Advertisement

D. NAT

Correct Answer: C. Router Advertisement

Question 5

What is the primary IPv6 replacement for broadcast delivery?

A. Unicast flooding

B. Multicast

C. PAT

D. DNS recursion

Correct Answer: B. Multicast

Question 6

Which statement about DHCPv6 is correct?

A. DHCPv6 always supplies the default gateway

B. DHCPv6 cannot provide DNS information

C. DHCPv6 may provide stateful addresses or additional configuration

D. DHCPv6 replaces NDP

Correct Answer: C. DHCPv6 may provide stateful addresses or additional configuration

Question 7

What is the purpose of Duplicate Address Detection?

A. To calculate a subnet mask

B. To determine whether an IPv6 address is already in use

C. To translate private addresses

D. To summarize routes

Correct Answer: B. To determine whether an IPv6 address is already in use

Question 8

Which IPv4 range is private?

A. 8.0.0.0/8

B. 100.64.0.0/10 only

C. 172.16.0.0/12

D. 192.0.2.0/24 only

Correct Answer: C. 172.16.0.0/12

Question 9

Why should an IPv6 firewall normally permit selected ICMPv6 traffic?

A. ICMPv6 is required for NDP and path MTU discovery

B. ICMPv6 encrypts all application traffic

C. ICMPv6 assigns IPv4 addresses

D. ICMPv6 performs NAT

Correct Answer: A. ICMPv6 is required for NDP and path MTU discovery

Question 10

What is the principal purpose of IPAM?

A. To replace all routing protocols

B. To manage address space, assignments, and related records

C. To compress IPv6 addresses on the wire

D. To convert DNS names into passwords

Correct Answer: B. To manage address space, assignments, and related records