IPvFooBar

Fork of IPvFoo (by pmarks.net).
New features: WHOIS, reverse DNS, ARIN and RIPE hyperlinks, fixes.
Uses Google DNS (dns.google) API for reverse DNS queries.

IPvFooBar is Free Software (Apache 2.0 license).

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Original functionality of IPvFoo:
Display the server IP address, with a realtime summary of IPv4, IPv6, and HTTPS information across all page elements.
The plugin uses the webRequest API to extract protocol-related information as a webpage downloads, and summarizes it into a convenient table.

The first thing you'll see is an icon in the location bar.  A large 4 or 6 shows whether the outer page was fetched using IPv4 or IPv6.  If the page contains elements from other domains, then smaller numbers will appear alongside for those.

When you click the icon, a table pops up, containing a row of information for each domain:

- A padlock icon, for HTTP, HTTPS, or a mix of both.  This is helpful for tracking down mixed content warnings, but you shouldn't treat it as absolute security advice.

- The IPv4 or IPv6 address.  If connections span more than one IP, then the most recent one wins.  When the connection is still open, the address is highlighted in yellow.

- A "cached" symbol.  This mainly exists to warn you that no actual connections took place, so the IP address might be stale.

- An "S" for WebSocket handshakes in Chrome 58+.