Source

File: wp-includes/Requests/IPv6.php

public static function check_ipv6($ip) {
		$ip = self::uncompress($ip);
		list($ipv6, $ipv4) = self::split_v6_v4($ip);
		$ipv6 = explode(':', $ipv6);
		$ipv4 = explode('.', $ipv4);
		if (count($ipv6) === 8 && count($ipv4) === 1 || count($ipv6) === 6 && count($ipv4) === 4) {
			foreach ($ipv6 as $ipv6_part) {
				// The section can't be empty
				if ($ipv6_part === '') {
					return false;
				}

				// Nor can it be over four characters
				if (strlen($ipv6_part) > 4) {
					return false;
				}

				// Remove leading zeros (this is safe because of the above)
				$ipv6_part = ltrim($ipv6_part, '0');
				if ($ipv6_part === '') {
					$ipv6_part = '0';
				}

				// Check the value is valid
				$value = hexdec($ipv6_part);
				if (dechex($value) !== strtolower($ipv6_part) || $value < 0 || $value > 0xFFFF) {
					return false;
				}
			}
			if (count($ipv4) === 4) {
				foreach ($ipv4 as $ipv4_part) {
					$value = (int) $ipv4_part;
					if ((string) $value !== $ipv4_part || $value < 0 || $value > 0xFF) {
						return false;
					}
				}
			}
			return true;
		}
		else {
			return false;
		}
	}