Writing
Working notes, not a blog
Each piece here starts from a failure that a calculator on this site was built to catch, and works through the arithmetic or the configuration by hand. They are written by Alex Xu and they are the reasoning behind the tools rather than an introduction to them.
- RoutingWhy a packet dies: three failures that look identical from the sourceNo route, no ARP reply, and two masks that disagree all produce the same symptom from the sending host. Here is how to tell them apart before you start changing configuration.21 October 2026 · 6 min read
- BGPWhat BGP summary-only actually hidesAn aggregate advertises address space, not the routes inside it. When the two do not match, summary-only turns a tidy configuration into a blackhole — and the routing table still looks correct.21 October 2026 · 5 min read
- AddressingHeadroom costs one bit of prefix and buys back a re-addressing projectSizing every VLAN to exactly what it needs today is the plan that survives until the first growth spurt. The arithmetic for thirty percent headroom, worked through end to end on a /21.21 October 2026 · 5 min read
- DatacenterWhy your PDU says 16 A and the breaker says 25Watts to amps is one division. Choosing the breaker is three, because a continuous load may only use eighty percent of it — and the outlet count is a third calculation again.21 October 2026 · 5 min read
- IPv6IPv6 reverse DNS only delegates on nibblesEvery hex digit of an IPv6 address is its own DNS label, which is why a /33 cannot be delegated and a /56 can — and why most operators never create the zones the standard allows.21 October 2026 · 6 min read
- EncapsulationThe three MTU numbers that decide whether a tunnel worksPhysical MTU, encapsulation overhead and the TCP MSS option are three different numbers, and a tunnel that ignores the second one produces the most misleading failure in networking: it connects, then stalls.21 October 2026 · 6 min read
- Addressing/31 on point-to-point links: where it works and where it quietly does notRFC 3021 halves the address cost of every transit link. The saving is real, and so are the four situations where a /31 is the wrong answer.21 October 2026 · 5 min read
Every number in these pieces can be reproduced with the calculator linked at the bottom of the article. If one of them is wrong, or a vendor detail is out of date, email alex.xu@ccie13192.com and the correction will be made on the page rather than in a reply.
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