[Sahana2524]

SubnetUtils6 turns its string input into bytes with InetAddress.getByName(), which falls back to a DNS lookup for anything that is not an IP literal. SubnetInfo.isInRange(String) and the SubnetUtils6(String) constructors are documented to take an IPv6 address, but today they also accept host names:

• new SubnetUtils6("2001:db8::/32").getInfo().isInRange("attacker.example") returns true when that name has an AAAA record in 2001:db8::/32. If an application checks a client-supplied value against an allowlist (for example a forwarded-for header), sending a name the client controls passes the check.
• Every such call also makes the server resolve a name chosen by the client, and "" is mapped to the loopback address.

The IPv4 SubnetUtils already parses with a regex and never resolves names, so this brings SubnetUtils6 in line with it. The new check only lets through strings shaped like an IPv6 literal (hex digits and a colon first, optional brackets and %scope). For those, getByName() always parses the literal and never does a lookup. Bracketed and scoped literals keep working.

Repro (JDK 18+ resolver SPI mapping attacker.example to 2001:db8::5), before and after:

isInRange("attacker.example") = true          # [DNS lookup] attacker.example
isInRange("attacker.example") -> IAE: Could not parse [attacker.example] - not an IPv6 address

About the test: testHostNameIsNotResolved always passes with the fix because no lookup happens. Without the fix it fails only where one of the names resolves to IPv6. That happens with ip6-localhost on typical Linux hosts, or with -DargLine=-Djava.net.preferIPv6Addresses=true, which is how I confirmed it locally. This touches the same toBytes() helper as #412 (IPv4-mapped literals). The two changes are independent, and mapped literals like ::ffff:192.0.2.1 still match the new pattern.

Thanks for your contribution to Apache Commons! Your help is appreciated!

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  Claude Code (Anthropic) found this issue and drafted the fix, the test and this description. I reviewed all of it before submitting.
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