Subdomain A Records — Routing Subdomains

Subdomain A Records

A subdomain A record maps a specific subdomain (like blog.example.com or api.example.com) to an IPv4 address. This is how you direct different services on your domain to different servers.

BIND Zone File Syntax

; Common subdomain layout
blog.example.com.       3600    IN    A    203.0.113.60
api.example.com.        3600    IN    A    203.0.113.70
staging.example.com.    3600    IN    A    203.0.113.80
admin.example.com.      3600    IN    A    203.0.113.90
cdn.example.com.        3600    IN    A    203.0.113.100

; Using relative names (within the zone file for example.com)
blog        3600    IN    A    203.0.113.60
api         3600    IN    A    203.0.113.70
staging     300     IN    A    203.0.113.80

When to Use Subdomain A Records vs. CNAME

Choose A records when:

• You control the target server and know its static IP address
• You need other record types (MX, TXT) at the same subdomain
• You want the fastest possible resolution (no extra CNAME lookup)

Choose CNAME when:

• The target is managed by a third party (CDN, SaaS platform) and the IP may change
• You want automatic updates when the target provider changes IPs
• There are no other record types needed at that subdomain

Common Subdomain Patterns

Subdomain	Purpose	Example Target
www	Main website	Web server or CDN
blog	Blog or content site	CMS server
api	API endpoints	API gateway or server
staging	Pre-production testing	Staging server
admin	Admin panel	Internal server
mail	Mail server	SMTP server
ftp	File transfer	FTP server
dev	Development environment	Dev server

Architectural Considerations

Each subdomain can point to an entirely different infrastructure:

• www.example.com → Vercel or Netlify (via CNAME)
• api.example.com → AWS EC2 instance (via A record)
• blog.example.com → WordPress on a VPS (via A record)
• mail.example.com → Mail server IP (via A record, referenced by MX)

This flexibility allows teams to choose the best hosting solution for each service independently.

TTL Strategy for Subdomains

• Production subdomains (www, api): Use longer TTLs like 3600 (1 hour) for stable endpoints
• Staging/dev subdomains: Use shorter TTLs like 300 (5 minutes) since IPs change more frequently
• Before migrations: Lower TTL well in advance of any planned IP changes

Security Considerations

• Dangling DNS records: If you decommission a server but leave the A record in place, an attacker could claim that IP address and serve content on your subdomain. Always remove DNS records when retiring services.
• Subdomain takeover: Be especially careful with CNAME records pointing to external services — if your account with the service expires, an attacker may claim the endpoint.
• Access control: Consider using different IPs for public-facing and internal-only subdomains, combined with firewall rules.