IPv4 and VLSM: verified source review

Chapter 7’s supplied notes and readable exercise headers are the starting sources. Handwritten answers were checked independently with Python’s ipaddress library. Chapter 8’s full text and original-resolution images were not supplied.

Rules used in these missions

IPv4 has 32 bits. /p reserves p leading network bits; the remaining 32 − p are host bits. A conventional block has 2^(32 − p) addresses; usable capacity is that size minus network and broadcast. The network is an aligned multiple of the block size; broadcast is the last address. Usable addresses are strictly between.

CIDR writes address/prefix without historical default classes. Equal-size subdivision uses one child prefix inside an explicitly stated parent: 2^(child − parent) child blocks. VLSM combines different aligned block sizes to match requirements. A gateway uses a usable address; it is not an extra address when a requirement already includes it.

For VLSM, order large needs first, choose sufficient blocks, check containment and overlap, and assign usable interfaces. A /30 link has two usable addresses. /31 and /32 require distinct rules and are not graded in this release.

Historical A/B/C terminology provides context. It neither selects a modern prefix nor proves how many subnets exist.

Addressing context

RFC 1918 private ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Other ranges are not automatically safe to assume public or assignable; special-use ranges exist. Public address space requires appropriate allocation. The source exercise addresses are mathematical examples, not permission to allocate those public networks.

APIPA uses IPv4 link-local 169.254/16; these packets are not routed to another subnet. Investigate the stated automatic configuration/DHCP problem rather than inventing a unique failure. 127/8 is internal loopback; 127.0.0.1 tests the host, not the external network.

Corrections and limits

In VLAN Exercise #6, 172.16.9.256 is invalid: an octet cannot exceed 255. LAN D may instead use aligned 172.16.10.0/26. The three /30 links and gateway-inclusive counts are explicit starter assumptions, not inferred from the picture. The valid sample is only one layout; any complete design satisfying capacity, alignment, containment, disjointness and interface constraints is accepted.

The Chapter 7 note’s /24 subnet count requires an explicit parent (for example 10.0.0.0/8). CIDR and VLSM are related, but not synonyms. The archive’s “CODEX-PROMPT.md” was not treated as an independent instruction.

⚠ image-2: 101 px displayed width; exercise headers and digits cannot all be transcribed reliably. Original image absent from ZIP.
⚠ image-4: 80 px displayed width; several headers/masks are ambiguous. Original image absent from ZIP.
⚠ image-6: 36 px displayed width; full strip is not reliably readable. Original image absent from ZIP.

Unreadable problems are excluded rather than reconstructed from guesses. Clearer originals are needed to finish their transcription.

Source and exam mapping

Network+ N10-009, objectives version 4.0: objective 1.7, IPv4 addressing/subnetting. Confirmed against CompTIA’s official objectives; no A+ or CCNA objective numbers are inferred. IPv6 remains separate review, not part of these IPv4 calculations.

Full transcription, independently calculated fixture table and audit trail: docs/subnet-source-verification.md in the repository. Original Chapter 7 source is preserved in content/sources/chapter-7-ip-addressing.rtf.

Optional IPv6 review · separate and unscored

No points or IPv4 mission progress are awarded here.

Separate review: how wide is an IPv6 address?

Which approach lets a node support both IPv4 and IPv6 protocol stacks?

RFC 4291: IPv6 addressing · RFC 4213: transition and dual stack

Return to practice and check understanding