diff --git a/httpx/_utils.py b/httpx/_utils.py
index 7fe827da4d..7c8820cda1 100644
--- a/httpx/_utils.py
+++ b/httpx/_utils.py
@@ -67,7 +67,13 @@ def get_environment_proxies() -> dict[str, str | None]:
             elif is_ipv4_hostname(hostname):
                 mounts[f"all://{hostname}"] = None
             elif is_ipv6_hostname(hostname):
-                mounts[f"all://[{hostname}]"] = None
+                if "/" in hostname:
+                    CIDR = hostname.split("/")
+                    hostname = f"{CIDR[0]}"
+                    subnet = f"/{CIDR[1]}"
+                else:
+                    subnet = ""
+                mounts[f"all://[{hostname}]{subnet}"] = None
             elif hostname.lower() == "localhost":
                 mounts[f"all://{hostname}"] = None
             else:
diff --git a/tests/test_utils.py b/tests/test_utils.py
index f9c215f65a..276db17290 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -100,6 +100,7 @@ def test_logging_redirect_chain(server, caplog):
         ({"no_proxy": "127.0.0.1"}, {"all://127.0.0.1": None}),
         ({"no_proxy": "192.168.0.0/16"}, {"all://192.168.0.0/16": None}),
         ({"no_proxy": "::1"}, {"all://[::1]": None}),
+        ({"no_proxy": "fe11::/16"}, {"all://[fe11::]/16": None}),
         ({"no_proxy": "localhost"}, {"all://localhost": None}),
         ({"no_proxy": "github.com"}, {"all://*github.com": None}),
         ({"no_proxy": ".github.com"}, {"all://*.github.com": None}),