A domain name can resolve to multiple addresses, a fundamental and crucial technology in modern internet architecture. This practice is widely used to improve website availability, distribute traffic load, and optimize access speed, among other scenarios.

When you type a domain name into your browser, the Domain Name System (DNS) translates that name into an IP address that computers can understand. Normally, a domain name corresponds to one IP address. However, the DNS protocol was designed from the outset to allow multiple A records (IPv4) or multiple AAAA records (IPv6) for a single domain name. This means you can tell the world, "To access my website, you can use any of these 'addresses'."

Why do this? There are three main uses. First, to achieve simple load balancing and redundancy. This is the most common use. When your website or service has a large user base, if all traffic floods a single server, it is likely to be overwhelmed and crash. By resolving a domain name to the IP addresses of multiple servers, traffic is distributed across these servers, thus relieving the pressure. This DNS-based load balancing is commonly known as "DNS Round Robin." Its working principle is very straightforward: when a user queries a domain name, the DNS server returns multiple IP addresses provided by the user in a round-robin fashion. For example, the first query might return `[IP1, IP2, IP3]`, and the second might return `[IP2, IP3, IP1]`. Since the client (usually the user's browser or operating system) typically tries to connect to the first IP address in the returned list, this round-robin approach macroscopically and evenly directs users to different servers. This also naturally creates redundancy. If one of the servers (e.g., IP2) goes down, as long as the user's client or the intermediate network has a retry mechanism, it will try to connect to the next IP in the list (e.g., IP3), thus ensuring uninterrupted service.

Secondly, it performs intelligent geolocation resolution. This is a more advanced usage, often called "intelligent DNS" or "resolution." Its core logic is not to randomly or in a round-robin fashion allocate IPs, but rather to return the IP address of the server closest to the user and with the fastest access speed, based on the user's location. For example, if your website has servers deployed in Beijing, Shanghai, and Guangzhou, when a user in Shenzhen accesses the site, the intelligent DNS system, by determining that their IP address belongs to the South China region, will prioritize returning the IP address of the Guangzhou server. Similarly, visitors from North America may be redirected to your servers on the US West Coast. This significantly reduces network latency and improves the access experience for users worldwide. One of the core technologies of major cloud service providers and CDN service providers is a global intelligent DNS resolution network.

Third, service separation and canary releases. You can also resolve different subdomains to different IP clusters to achieve physical separation of services. For example, resolve `api.example.com` to the IP of the backend API server cluster, and resolve `static.example.com` to the IP of an object storage server specifically for storing static resources such as images and CSS. Furthermore, during product updates, you can perform canary releases or A/B testing by resolving the domains of a small number of users to the new version's server IP (e.g., through a specific DNS resolution strategy), while the majority of users still access the stable version.

How to configure? The specific configuration for both main methods is done in your domain's DNS management interface. You need to add multiple A records (or AAAA records) that have the same host record (e.g., `@` represents the root domain, or `www`), but point to different IP address values.

One approach is the basic DNS round-robin configuration. In your DNS control panel, add multiple A records for the same hostname. For example, to point `www.example.com` to three IPs:

Type: A | Host Record: www | Record Value: 192.0.2.1
Type: A | Host Record: www | Record Value: 192.0.2.2
Type: A | Host Record: www | Record Value: 192.0.2.3

The DNS server will automatically round these records when responding to queries.

Another approach is through Smart Resolution (geolocation-based). This usually has intuitive options in the console of a professional DNS provider. You need to create multiple resolution lines for the same hostname and specify the IP address for each line. The configuration might look like this:

Line: China Mainland - China Telecom | Record Type: A | Host Record: www | Record Value: 203.0.113.1

Line: China Mainland - China Unicom | Record Type: A | Host Record: www | Record Value: 203.0.113.2

Line: Overseas - Default | Record Type: A | Host Record: www | Record Value: 198.51.100.1

The system will automatically return the IP address corresponding to the line based on the queryer's network attributes.

While multi-IP resolution is very useful, the following issues need to be considered during implementation to ensure its effectiveness and stability.

Basic DNS round-robin has a major flaw: it lacks health check capabilities. If the server `192.0.2.2` is down, the DNS record will still return it to the user, causing access failure for those users. Solving this problem requires using a more advanced DNS service. Many cloud DNS providers offer a "DNS health check" feature, which periodically probes your server ports (such as 80 or 443). If a server is found to be unresponsive, its IP address is automatically and temporarily removed from the DNS resolution results.

DNS resolution results are cached at multiple levels, including the user's local network and the ISP's DNS servers. The parameter controlling the caching duration is called TTL (Time To Live). If you set a very long TTL (e.g., 7200 seconds, or 2 hours), then if you need to change an IP address or take a server offline, it could take up to 2 hours for users worldwide to refresh to the new, correct DNS records. Therefore, when using multiple IP resolutions, especially for services with high availability requirements, it is recommended to set a shorter TTL (e.g., 300 seconds, or 5 minutes). This allows changes to take effect faster, but it slightly increases the burden of DNS queries.

DNS-based round-robin is a very "coarse" load balancing method. It cannot perceive the real-time load (CPU, memory usage) of backend servers, nor can it guarantee session persistence. For example, multiple requests from a single user within a session might be distributed across different servers due to DNS round-robin or client retries. If application state is stored in local memory, this could lead to session loss. Therefore, for complex, stateful applications, a dedicated load balancer (such as Nginx, HAProxy, or a cloud provider's LB product) is typically deployed in front of the DNS multi-IP resolution to receive all traffic and perform fine-grained distribution and health checks.

If your website uses HTTPS, the certificate's alias must cover the domain name you are using. Whether it's `example.com` or `www.example.com`, the certificate must be valid. As long as the certificate is correctly installed and configured on all backend servers, multi-IP resolution will not introduce additional certificate issues. Using a wildcard certificate (such as `*.example.com`) makes it easier to cover all subdomains.

In today's network environment where IPv4 and IPv6 coexist, best practice is to configure both A records (IPv4) and AAAA records (IPv6) for your domain. Clients that support IPv6 will prioritize querying the AAAA record. You can configure multiple IPv4 addresses and multiple IPv6 addresses for the same group of services, thereby achieving load balancing and high availability with dual protocol stacks.

In short, resolving a domain name to multiple IP addresses is a classic and powerful technique. It provides fundamental support for service scalability, reliability, and performance optimization at the internet's underlying DNS layer. Understanding its principles and applying it properly, combined with health checks, appropriate TTL, and higher-level load balancing technologies, allows you to build a robust and efficient network service architecture.