The Domain Name System (DNS) technically allows up to 127 levels of subdomain nesting according to RFC 1035, with each label (subdomain segment) limited to 63 characters and total FQDN length limited to 253 characters. However, real-world implementations often impose practical limits.

// Example of deeply nested subdomain in DNS configuration
$ORIGIN com.
google IN NS ns1.google.com.
pirate IN NS ns1.google.com.
monkey IN NS ns1.google.com.
baz IN NS ns1.google.com.
bar IN NS ns1.google.com.
foo IN NS ns1.google.com.

Popular web servers have varying behaviors:

• Apache: No hard-coded limit, but performance degrades with complex regex matching in VirtualHosts
• Nginx: Default server_name hash buckets may need adjustment for long domains
• IIS: Maximum 255 characters total for host headers

Modern browsers generally handle long subdomains well, but we've observed:

// JavaScript domain parsing example
let domain = "a.".repeat(50) + "example.com";
console.log(new URL(http://${domain}).hostname); 
// Most browsers will handle this, but cookies may fail

Several web standards impose functional limits:

• Cookies: RFC 6265 suggests 4096 bytes total per cookie, including domain attributes
• TLS/SSL Certificates: Most CAs limit certificates to 100 characters per label
• HTTP Headers: Typically limited to 8KB total per request

1. Keep nesting to 3-4 levels maximum for maintainability
2. Monitor performance when using regex-based vhost matching
3. Test cookie behavior across subdomains early in development

// Node.js example for safe subdomain handling
const MAX_SUBDOMAIN_DEPTH = 5;

function validateSubdomain(hostname) {
  const parts = hostname.split('.').filter(Boolean);
  if (parts.length > MAX_SUBDOMAIN_DEPTH + 2) {
    throw new Error('Subdomain depth exceeds maximum allowed');
  }
  return true;
}

Technically, DNS specifications (RFC 1035) allow domain names up to 253 characters total length, with each label (subdomain) limited to 63 characters. There's no explicit limit on nesting depth, but practical constraints emerge from:

// Example of DNS query for deep subdomain
nslookup level1.level2.level3.level4.example.com

Major web servers impose varying constraints:

• Apache: 127 subdomains by default (MAXDNAME in apr-util)
• Nginx: No hardcoded limit, but performance degrades with depth
• IIS: 250 character total limit for binding names

Testing reveals browser-specific behaviors:

// JavaScript domain parsing test
let domain = 'a'.repeat(50).split('').join('.') + '.example.com';
console.log(new URL(http://${domain}).hostname);

Chrome 105+ limits to 40 labels, Firefox allows ~60, while Safari truncates at 35.

Deep subdomains affect cookie handling:

// Setting cross-subdomain cookie
document.cookie = "test=1; domain=.level1.level2.example.com; path=/";

Most browsers reject cookies with >5 dots in domain attribute due to security policies.

Each subdomain level adds:

• ~100ms DNS lookup overhead (without preconnect)
• Additional TLS negotiation cycles
• Complexity in certificate wildcard matching

For production systems:

# Recommended nginx configuration
server_name ~^(?.+)\.example\.com$;
if ($subdomain =~ "\.") {
    return 444; # Block excessively deep subdomains
}