diff --git a/Cargo.toml b/Cargo.toml
index 350f5f1..ac8e9d2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,6 +5,7 @@ edition = "2024"
 
 [dependencies]
 axum = "0.8.8"
+tower = { version = "0.5", features = ["util"] }
 tokio = { version = "1.49.0", features = ["rt-multi-thread", "macros", "net", "signal", "time", "fs", "io-util"] }
 askama = "0.15.4"
 maxminddb = "0.27.3"
diff --git a/src/handlers.rs b/src/handlers.rs
index 15631fe..2be69dc 100644
--- a/src/handlers.rs
+++ b/src/handlers.rs
@@ -656,6 +656,45 @@ pub async fn health() -> StatusCode {
     StatusCode::OK
 }
 
+fn extract_ipv4_client_ip(headers: &HeaderMap, dev_mode: bool) -> Result<IpAddr, AppError> {
+    if let Some(ip) = headers
+        .get("X-IPv4-Client-IP")
+        .and_then(|v| v.to_str().ok())
+        .and_then(|s| s.trim().parse().ok())
+    {
+        return Ok(ip);
+    }
+
+    if dev_mode {
+        return Ok(DEV_FALLBACK_IP);
+    }
+
+    Err(AppError::MissingClientIp)
+}
+
+pub async fn ipv4_ip_handler(
+    State(state): State<AppState>,
+    headers: HeaderMap,
+) -> Result<Response, AppError> {
+    let ip = extract_ipv4_client_ip(&headers, state.dev_mode)?;
+    let origin = format!("https://{}", state.site_domain);
+    Ok((
+        [
+            (header::ACCESS_CONTROL_ALLOW_ORIGIN, origin),
+            (
+                header::ACCESS_CONTROL_ALLOW_METHODS,
+                "GET".to_owned(),
+            ),
+            (
+                header::HeaderName::from_static("access-control-max-age"),
+                "86400".to_owned(),
+            ),
+        ],
+        format!("{ip}\n"),
+    )
+        .into_response())
+}
+
 pub async fn json_handler(
     State(state): State<AppState>,
     headers: HeaderMap,
@@ -915,3 +954,7 @@ pub async fn not_found(State(state): State<AppState>, headers: HeaderMap) -> Res
         "The page you are looking for does not exist.",
     )
 }
+
+pub async fn ipv4_not_found() -> StatusCode {
+    StatusCode::NOT_FOUND
+}
diff --git a/src/middleware.rs b/src/middleware.rs
index 3bf5a14..8f91dbe 100644
--- a/src/middleware.rs
+++ b/src/middleware.rs
@@ -5,8 +5,29 @@ use axum::response::Response;
 
 pub async fn security_headers(request: Request, next: Next) -> Response {
     let mut response = next.run(request).await;
-    let headers = response.headers_mut();
+    apply_common_security_headers(response.headers_mut());
+    response.headers_mut().insert(
+        header::HeaderName::from_static("cross-origin-opener-policy"),
+        HeaderValue::from_static("same-origin"),
+    );
+    response.headers_mut().insert(
+        header::HeaderName::from_static("cross-origin-resource-policy"),
+        HeaderValue::from_static("same-origin"),
+    );
+    response
+}
 
+pub async fn ipv4_domain_security_headers(request: Request, next: Next) -> Response {
+    let mut response = next.run(request).await;
+    apply_common_security_headers(response.headers_mut());
+    response.headers_mut().insert(
+        header::HeaderName::from_static("cross-origin-resource-policy"),
+        HeaderValue::from_static("cross-origin"),
+    );
+    response
+}
+
+fn apply_common_security_headers(headers: &mut axum::http::HeaderMap) {
     headers.insert(
         header::X_CONTENT_TYPE_OPTIONS,
         HeaderValue::from_static("nosniff"),
@@ -28,14 +49,4 @@ pub async fn security_headers(request: Request, next: Next) -> Response {
         header::HeaderName::from_static("permissions-policy"),
         HeaderValue::from_static("camera=(), microphone=(), geolocation=(), interest-cohort=()"),
     );
-    headers.insert(
-        header::HeaderName::from_static("cross-origin-opener-policy"),
-        HeaderValue::from_static("same-origin"),
-    );
-    headers.insert(
-        header::HeaderName::from_static("cross-origin-resource-policy"),
-        HeaderValue::from_static("same-origin"),
-    );
-
-    response
 }
diff --git a/src/routes.rs b/src/routes.rs
index 6d43331..bec1f2a 100644
--- a/src/routes.rs
+++ b/src/routes.rs
@@ -1,12 +1,16 @@
+use std::sync::Arc;
+
 use axum::Router;
 use axum::middleware;
+use axum::response::IntoResponse;
 use axum::routing::get;
+use tower::ServiceExt as _;
 
 use crate::handlers::{self, AppState};
-use crate::middleware::security_headers;
+use crate::middleware::{ipv4_domain_security_headers, security_headers};
 use crate::static_assets;
 
-pub fn build_router(state: AppState) -> Router {
+fn build_main_router(state: AppState) -> Router {
     Router::new()
         .route("/", get(handlers::root_redirect))
         .route("/favicon.ico", get(handlers::favicon))
@@ -32,3 +36,45 @@ pub fn build_router(state: AppState) -> Router {
         .layer(middleware::from_fn(security_headers))
         .with_state(state)
 }
+
+fn build_ipv4_router(state: AppState) -> Router {
+    Router::new()
+        .route("/", get(handlers::ipv4_ip_handler))
+        .route("/health", get(handlers::health))
+        .fallback(handlers::ipv4_not_found)
+        .layer(middleware::from_fn(ipv4_domain_security_headers))
+        .with_state(state)
+}
+
+pub fn build_router(state: AppState) -> Router {
+    let ipv4_domain: Arc<str> = state.ipv4_domain.clone();
+    let main_router = build_main_router(state.clone());
+    let ipv4_router = build_ipv4_router(state);
+
+    let svc = tower::service_fn(move |req: axum::http::Request<axum::body::Body>| {
+        let ipv4_domain = ipv4_domain.clone();
+        let main_router = main_router.clone();
+        let ipv4_router = ipv4_router.clone();
+        async move {
+            let is_ipv4_domain = !ipv4_domain.is_empty()
+                && req
+                    .headers()
+                    .get("host")
+                    .and_then(|v| v.to_str().ok())
+                    .is_some_and(|host| {
+                        let host = host.split(':').next().unwrap_or(host);
+                        host.eq_ignore_ascii_case(&ipv4_domain)
+                    });
+
+            // Router<()> is an infallible service; unwrap is safe.
+            let response = if is_ipv4_domain {
+                ipv4_router.oneshot(req).await.into_response()
+            } else {
+                main_router.oneshot(req).await.into_response()
+            };
+            Ok::<_, std::convert::Infallible>(response)
+        }
+    });
+
+    Router::new().fallback_service(svc)
+}