Network terms and practical checks
Read each term through its meaning, an example, the limits of a conclusion and the next useful check.
Instructions, guides and related tools
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Information sent when visiting a site
A web connection exposes a source IP and HTTP request information to the receiving service. Screen size and time zone are separate information that page JavaScript can read.
Example
Accept-Language describes language preferences; it does not prove nationality or location.
What this does not establish
The IP page limits displayed fields and excludes Cookie and authentication headers. Browser-reported values can also be changed.
What to check next
Compare connection information with browser information on the IP page. Remove unnecessary values before sharing.
Primary sources: RFC 9110 — HTTP semantics
IP addresses, IPv4 and IPv6
An IP address identifies a network source or destination. IPv4 and IPv6 use different formats; one device may use both.
Example
192.168.1.20 is a private IPv4 example. A website may see your router or VPN's public address instead.
What this does not establish
A shared public IP does not identify a person or device. One connection cannot establish reachability over both address families.
What to check next
Check the address family on the IP page, compare device settings, and check the VPN route if relevant.
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Primary sources: RFC 1918 — Private IPv4 addresses · RFC 8200 — IPv6
Domain names and management
A domain name contains labels separated by dots, with the hierarchy read from the right. A DNS name is not always a separate registration.
Example
In www.example.com, com is the TLD, followed by example and then www. Do not infer the registration boundary by always taking two labels.
What this does not establish
Registration, DNS hosting and web hosting can be managed separately. A registered name need not have a website.
What to check next
Identify your registration provider, configured name servers and web destination separately.
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Primary sources: RFC 1034 — Domain names
Internationalized names and Punycode
Internationalized domain names map Unicode labels to ASCII labels beginning with xn--. Conversion operates on individual labels.
Example
The 日本語 label in 日本語.jp becomes xn--wgv71a119e. This differs from percent-encoding a URL path.
What this does not establish
Similar-looking characters may identify different names. Successful conversion does not prove registration eligibility.
What to check next
Check the ASCII form before configuring DNS or certificates, and check registration rules with your provider.
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Primary sources: RFC 5890 — IDNA labels
DNS, caching and TTL
DNS retrieves records associated with names. Authoritative servers hold zone data; recursive resolvers look up and may cache answers.
Example
A holds IPv4, AAAA IPv6, and MX mail destinations. TTL concerns cache lifetime in seconds, not lookup duration.
What this does not establish
NXDOMAIN, empty NOERROR and SERVFAIL have different meanings. Agreement between two resolvers does not prove global propagation.
What to check next
Compare the changed record type, resolver, observation time and TTL.
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Primary sources: RFC 9499 — DNS terminology
WHOIS, RDAP and registration data
WHOIS and RDAP provide registration information. RDAP uses structured JSON for fields such as events and status.
Example
Registration, update and expiration are different events. Check event labels and sources; IP allocation data is separate from domain registration.
What this does not establish
Failure or redaction does not mean unregistered. Published fields vary, and expiration does not imply immediate availability.
What to check next
Confirm renewal and ownership in your registrar account, not solely from a public lookup.
Primary sources: RFC 9083 — RDAP responses
Reading a URL
A URL identifies a resource using components such as scheme, host, path and query. It contains more than a domain name.
Example
In https://example.com/help?lang=ja#answer, the host is example.com, the path /help, and the fragment identifies a position within the resource.
What this does not establish
Queries can contain authentication tokens. Review the complete URL before sharing it.
What to check next
For a single failing page, record the path and HTTP/HTTPS scheme as well as the host.
Primary sources: RFC 3986 — URI syntax
HTTP, status codes and redirects
HTTP exchanges requests and responses. Status codes and headers help explain how a server handled a request after connecting.
Example
301/302 indicate redirects, 403 refusal, and 404 a missing target. A later redirect destination can fail.
What this does not establish
A 200 response can contain a login page or error message. No HTTP response is not a 404.
What to check next
Use browser developer tools to inspect the failing URL, response status and redirect destination.
Primary sources: RFC 9110 — HTTP semantics
SSL, TLS and certificates
Modern HTTPS uses TLS for encryption and peer authentication. The common name SSL should not be confused with obsolete SSL protocols.
Example
Check the certificate's names, validity and trust chain. An unexpired certificate for another host is still a problem.
What this does not establish
One negotiated protocol and cipher cannot reveal all server capabilities. A valid certificate does not establish trustworthy site content.
What to check next
Check the warning and device clock. Report the hostname and time instead of bypassing a warning to sign in.
Primary sources: RFC 8446 — TLS 1.3
Ping, ICMP and round-trip time
Ping uses ICMP requests and replies to observe reachability and round-trip time. It does not measure page download time or bandwidth.
Example
One missing reply out of four is 25% loss in that short observation; it does not establish long-term quality.
What this does not establish
A site may work while ICMP is filtered. No reply alone does not prove the server is down.
What to check next
Use a bounded check of your own gateway and record it separately from web symptoms.
Primary sources: Microsoft: ping
Port numbers, TCP and UDP
Port numbers distinguish application endpoints within TCP or UDP. The same number in TCP and UDP represents separate endpoints.
Example
HTTPS commonly uses TCP 443; HTTP/3 uses UDP. A port number alone does not identify the actual service.
What this does not establish
A TCP connection does not prove login or page success. Filtering or routing can also cause timeouts.
What to check next
Check the required protocol and exposure for one managed port. Do not start by exposing an entire router or host.
Primary sources: IANA: Service names and port numbers · RFC 9114 — HTTP/3
Subnets and CIDR
The number after / in CIDR is the network prefix length in bits. It describes an address range, not connection speed.
Example
192.168.1.0/24 contains 256 addresses from .0 through .255. Not all necessarily serve as assignable host addresses.
What this does not establish
/32 identifies one IPv4 address; /128 one IPv6 address. Sharing a subnet does not establish permission to communicate.
What to check next
Use the necessary range for IP restrictions and check the address, mask and gateway on the device.
Primary sources: RFC 4632 — CIDR · RFC 8200 — IPv6
MAC addresses
A MAC address is a link-layer identifier used by technologies such as Ethernet and Wi-Fi. Its role differs from an IP address.
Example
A device with several network adapters has separate addresses and may use private or randomized Wi-Fi addresses.
What this does not establish
A remote website normally does not receive the device's MAC address. A vendor prefix cannot identify its user.
What to check next
Inspect device network settings rather than expecting a public IP website to report a MAC address.
Primary sources: RFC 9542 — IEEE 802 parameters
DHCP and automatic configuration
DHCP distributes address and network configuration. IPv4 DHCP assigns addresses with a lease lifetime.
Example
A device may join Wi-Fi but fail to obtain the address or gateway needed for internet access.
What this does not establish
IPv6 can also use router advertisements; a DHCP field alone does not establish IPv6 connectivity.
What to check next
On Windows, inspect configuration with ipconfig /all. Releasing a lease can disconnect a remote session.
Primary sources: RFC 2131 — DHCP · Microsoft: ipconfig
VPNs, routes and public IPs
A VPN creates a logical connection to another network. Encryption and traffic scope depend on its protocol and configuration.
Example
Full-tunnel and split-tunnel configurations can produce different public-IP behavior for web browsing.
What this does not establish
Applications may use different routes or DNS settings even with a VPN connected. An IP change does not prove anonymity.
What to check next
Compare IP and DNS settings only as allowed by your organization. Do not bypass a required VPN.
Check pages to open next
Primary sources: RFC 4301 — IPsec architecture · Microsoft: VPN routing decisions
NAT, NAPT and router DMZ settings
NAT translates addresses; NAPT also uses ports to map multiple connections. Translation can occur at both home and provider networks.
Example
Several devices may share one public IPv4 address. A private WAN address can indicate another upstream translation.
What this does not establish
A consumer router's DMZ-host option may differ from an isolated network DMZ. The label does not establish security.
What to check next
For an intended public service, check its specific port and provider NAT. Avoid using blanket port forwarding as a troubleshooting shortcut.
Primary sources: RFC 3022 — NAT and NAPT
Find troubleshooting steps by symptom
FAQ
Are IP and MAC addresses the same?
No. IP supports network communication; MAC operates at the link layer, such as Ethernet or Wi-Fi. A remote website normally does not receive your device's MAC address.
Does TTL mean the same thing in DNS and ping?
DNS TTL concerns caching in seconds. The IP-header TTL decreases during forwarding; the value printed by ping does not describe DNS propagation time.
Does a passing check guarantee the service works?
Every check has a scope. A DNS answer, TCP connection, TLS authentication, HTTP response and successful login are separate stages. Identify the stage affected by the symptom.
Next checks and services
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