Internet Tools

HTTP/2 and HTTP/3 changed the transport layer — binary framing, header compression, multiplexing, QUIC — but the headers you see in a response inspector are mostly the same HTTP/1.1 headers. Here's what HTTP/2's HPACK compression and multiplexing actually do, what the :method/:path/:status pseudo-headers mean, how to identify which HTTP version served a response, and what the Alt-Svc h3 header signals about HTTP/3 availability.

"DNS propagation takes up to 48 hours" is true but vague — the actual mechanism is TTL-governed caching at thousands of independent resolvers, each expiring on its own schedule. Here's how TTL actually controls this, why lowering TTL before a planned change matters, negative caching for new records, and why different users can genuinely see different DNS results during a transition.

A clean public blacklist record doesn't guarantee inbox delivery — Gmail, Outlook, and Yahoo operate private reputation systems that aren't published, don't have external lookup tools, and use engagement signals (open rates, spam complaint rates, deletes without opening) rather than just IP status. Here's the two-tier reputation system, what Google Postmaster Tools and Microsoft SNDS provide, the IP warm-up requirement, and why clearing DNSBLs is necessary but not sufficient.

Learn what NS records are, how DNS delegation works, why NS records are the first thing to check when DNS fails, and how to use a free NS lookup tool to verify nameserver configuration.

WHOIS records reveal domain age, ownership history, expiry dates, and registration patterns that matter for SEO domain research, vendor due diligence, and operational monitoring — not just availability checking.

Your domain can land on a spam blacklist without sending a single spam email — if you're on shared hosting, the IP address you share with hundreds of other sites carries one collective reputation, and someone else's compromised WordPress site becomes your deliverability problem. Here's how to recognize shared-IP contamination, what your hosting provider can (and you can't) do about it, and why "IP warming" matters when switching to a dedicated IP.

DKIM signatures stay valid indefinitely on unmodified messages — which means an attacker who receives a legitimately signed email can replay it to other recipients and the signature will still verify. Here's how DKIM replay attacks work, why the specification doesn't prevent them, what DMARC aggregate reports reveal about replay patterns, and the countermeasures (signing the To: header, signature expiry x= tag) that limit the attack surface.

Learn how DMARC works, what the none/quarantine/reject policies mean, how to read a DMARC record, and how to use a free DMARC lookup to diagnose email authentication and spoofing protection.

A /24 gives 254 usable hosts, not 256 — because subnetting always reserves the first address (network) and last address (broadcast). Here's the 2^n−2 formula across all prefix lengths, VLSM (Variable Length Subnet Masking) for efficiently dividing address space into different-sized subnets, why route summarization (supernetting) is the reverse operation, and what the slash number in CIDR notation actually means in binary.

SPF flattening solves the 10-lookup PermError by replacing include: mechanisms with their resolved IP addresses — but silently breaks every time a provider updates their IP ranges, which happens regularly without notification. Here's why SPF records go stale without anyone touching them, the specific maintenance trap of flattened records, why the ptr: mechanism should never be used, and how DMARC aggregate reports provide the best automated SPF monitoring.