The DNS specification (RFC 1035) doesn't explicitly limit the number of subdomains you can create under a domain. However, practical limitations exist due to:

• DNS protocol constraints (253-character total limit for a fully qualified domain name)
• Server configuration limits
• Performance considerations

Here's how different DNS providers handle subdomains:

// AWS Route 53 example (no hard limit documented)
aws route53 change-resource-record-sets \
--hosted-zone-id Z1PA6795UKMFR9 \
--change-batch file://subdomains.json

// Google Cloud DNS (supports up to 10,000 records per zone)
gcloud dns record-sets create sub.example.com. \
--type=A --zone=example-zone \
--rrdatas=192.0.2.1 --ttl=300

The 253-character limit applies to the complete domain name including all subdomains and dots. For example:

• Valid: a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z.example.com (total length: 253 chars)
• Invalid: any domain exceeding this length

While you can technically create thousands of subdomains, consider:

// Python DNS lookup performance test
import dns.resolver
import time

def test_lookup(domain):
    start = time.time()
    dns.resolver.resolve(domain, 'A')
    return time.time() - start

# Results show lookup times increase with subdomain depth

For large-scale subdomain implementations:

• Use wildcard records (*.example.com) when appropriate
• Consider DNS partitioning for very large deployments
• Monitor DNS query performance

When hitting practical limits:

// Nginx configuration for handling many subdomains
server {
    listen 80;
    server_name ~^(?.+)\.example\.com$;
    root /sites/$subdomain;
}

// Alternative using URL paths instead of subdomains
// example.com/user123 instead of user123.example.com

In DNS architecture, the theoretical maximum subdomain depth is 127 levels with each label (subdomain segment) limited to 63 characters. The total FQDN (Fully Qualified Domain Name) length cannot exceed 253 characters including dots. This means patterns like "a.b.c...domain.com" have hard technical constraints.

// Node.js DNS lookup test
const dns = require('dns');

const testSubdomain = 'a'.repeat(63) + '.' + 
                     'b'.repeat(63) + '.' +
                     'c'.repeat(63) + '.domain.com';

dns.resolve(testSubdomain, (err) => {
  if(err) console.log(DNS resolution error: ${err});
  else console.log('Valid subdomain structure');
});

While DNS allows deep nesting, web servers impose additional limits. For Nginx:

server {
    server_name ~^(?.+)\.domain\.com$;
    
    # Process up to 10 subdomain levels
    if ($subdomain ~* "^([^.]+\.){10}") {
        return 403;
    }
    
    # ... other configurations
}

SSL certificates (like Let's Encrypt) only cover one level of wildcards:

*.domain.com  # Valid
*.*.domain.com  # Invalid for most CAs

Python's socket module demonstrates hostname validation:

import socket

def validate_subdomain(sub):
    try:
        socket.gethostbyname(f"{sub}.domain.com")
        return True
    except socket.gaierror:
        return False

When storing subdomains in databases, consider efficient indexing strategies:

-- PostgreSQL reverse pattern indexing
CREATE INDEX idx_reverse_subdomain ON domains 
(REVERSE(subdomain) varchar_pattern_ops);

Benchmark different subdomain depths with tools like ab:

ab -n 1000 -c 10 http://test.domain.com/
ab -n 1000 -c 10 http://a.b.c.d.e.f.test.domain.com/