IPAddr#to_s Encoding Quirk in Ruby 3

Ruby’s IPAddr#to_s returns inconsistent encodings: IPv4 addresses use US-ASCII, IPv6 addresses use UTF-8.

Behavior

ruby IPAddr.new('192.168.1.1').to_s.encoding # => #<Encoding:US-ASCII> IPAddr.new('::1').to_s.encoding # => #<Encoding:UTF-8>

Cause

Different string construction in IPAddr’s _to_string method:

• IPv4: Array#join('.') → US-ASCII optimization
• IPv6: String#% → UTF-8 default

Impact

• Rack expects UTF-8 strings
• Mixed encodings cause Encoding::CompatibilityError
• String operations fail on encoding mismatches

Solution

Use force_encoding('UTF-8') instead of encode('UTF-8'):

• IP addresses contain only ASCII characters
• ASCII bytes are identical in US-ASCII and UTF-8
• force_encoding changes label only (O(1))
• encode creates new string (O(n))

This ensures consistent UTF-8 encoding across all IP strings.