diff --git a/changelog.d/265.changed.md b/changelog.d/265.changed.md
new file mode 100644
index 0000000..58e8e85
--- /dev/null
+++ b/changelog.d/265.changed.md
@@ -0,0 +1,6 @@
+**The host in an HTTP cache key is normalised.** It is lower-cased, and an
+empty port or the scheme's default one (`:80` for http, `:443` for https) is
+dropped, so `Example.com`, `example.com:80` and `example.com` share one entry
+instead of three. IPv6 literals keep their brackets. Keys for hosts written
+with upper case or a default port change, so those entries are cached afresh
+once.
diff --git a/docs/CACHE_FLOW.md b/docs/CACHE_FLOW.md
index 34756bf..40a6efb 100644
--- a/docs/CACHE_FLOW.md
+++ b/docs/CACHE_FLOW.md
@@ -72,7 +72,7 @@ cache_key = "|".join(
     [
         CacheKey.FORMAT_TAG,  # "http:v2"
         escape_key_component(request.method),
-        escape_key_component(request.headers.get("host", "unknown")),
+        escape_key_component(host),  # Host header, normalised (see below)
         escape_key_component(request.url.path),
         query,
     ]
@@ -120,7 +120,8 @@ The key format keeps each dimension cached independently:
 
 - **Method isolation**: GET and POST do not share a cache (and currently only GET
   enters the cache flow at all)
-- **Host isolation**: `example.com` and `api.example.com` are cached separately
+- **Host isolation**: `example.com` and `api.example.com` are cached separately;
+  `Example.com` and `example.com:80` (on http) are `example.com`
 - **Path isolation**: each endpoint has its own entries
 - **Query parameter isolation**: different query parameters on the same endpoint
   are cached separately
diff --git a/docs/HTTP_CACHING.md b/docs/HTTP_CACHING.md
index 9591884..83d51a7 100644
--- a/docs/HTTP_CACHING.md
+++ b/docs/HTTP_CACHING.md
@@ -340,9 +340,19 @@ stored key, so write `%7C` there for a `|`. Before 0.3.8 both were stored as sen
 so after upgrading, entries for a host or path containing `|` or `%` are cached
 afresh once.
 
-The host is still whatever the client sends. Unless a reverse proxy or load
-balancer in front of the app already rejects unknown hosts, add Starlette's
-`TrustedHostMiddleware`, so a forged `Host` gets a `400` instead of filling the
+The host is normalised first, so every spelling of one origin shares an entry:
+it is lower-cased (hostnames are case-insensitive), and an empty port or the
+scheme's default one (`:80` for http, `:443` for https) is dropped.
+`Example.com`, `example.com:80` and `example.com` on http are one key,
+`example.com`; `example.com:8080` keeps its port, and an IPv6 literal keeps its
+brackets (`[::1]:8000`). The scheme is the one the app sees: behind a proxy that
+terminates TLS it is `http` unless the proxy's headers are applied (for
+example `uvicorn --proxy-headers`), so a `Host: example.com:443` from such a
+proxy keeps its port. A request without a `Host` header uses `unknown`.
+
+Otherwise the host is still whatever the client sends. Unless a reverse proxy
+or load balancer in front of the app already rejects unknown hosts, add
+Starlette's `TrustedHostMiddleware`, so a forged `Host` gets a `400` instead of filling the
 cache with entries no one else will request:
 
 ```python
diff --git a/docs/MIGRATING_0_4.md b/docs/MIGRATING_0_4.md
index b4e4709..a18282c 100644
--- a/docs/MIGRATING_0_4.md
+++ b/docs/MIGRATING_0_4.md
@@ -252,7 +252,7 @@ CacheManagerProxy.set(CacheManager(lock=True))
 
 - The separator becomes a single `|` (`CACHE_KEY_SEPARATOR`).
 - Keys start with the format tag `http:v2|` (`CacheKey.FORMAT_TAG`), so the next format change can remove old keys by pattern: `clear_pattern("http:v2|*")` removes every key of this format.
-- The host is normalised: lower-cased, and the scheme's default port (`:80`, `:443`) dropped.
+- The host is normalised: lower-cased, and an empty port or the scheme's default one (`:80` on http, `:443` on https) dropped.
 - A long query string (over about 200 bytes) is stored as `sha256:` and its hex digest; the path stays readable.
 - Query parameters are sorted by name: `sort_query` (opt-in since 0.3.9) defaults to `True` in `@cache`, `build_cache_key()` and `invalidate()`, so `?b=2&a=1` and `?a=1&b=2` share one entry.
 - One public `CacheKey` type builds, encodes and parses keys. `CACHE_KEY_MIN_PARTS`, `CACHE_KEY_MAX_SPLIT` and `CACHE_KEY_MAX_PARTS` are removed from `fastapi_cachex.routes`; read a key's components with `CacheKey.parse(key)` instead.
@@ -264,7 +264,7 @@ After:  http:v2|GET|example.com|/users/1|page=2
 
 What to change:
 
-- `clear_pattern()` patterns that spell out the separator (`"GET|||*|||/users/*"`) need rewriting. `clear_path()` and `invalidate()` build the key themselves and need nothing.
+- `clear_pattern()` patterns that spell out the separator (`"GET|||*|||/users/*"`) need rewriting, and so do patterns that name a host in upper case or with a default port. `clear_path()` and `invalidate()` build the key themselves and need nothing.
 - A custom `key_builder` that calls `build_cache_key()` follows automatically. One that builds the key itself (joining with `CACHE_KEY_SEPARATOR` or hard-coding `|||`) still caches and still works with `invalidate()` and `clear_pattern()`, but its keys lack the `http:v2` tag, so `clear_path()` no longer finds them and the monitoring routes no longer list them. Switch it to `build_cache_key(request, *components)` to keep both.
 - A handler whose response depends on the order of the query string as sent, such as a self or pagination link copied from `request.url` or a signature over the raw query, should set `@cache(sort_query=False)`. Otherwise the first caller's order is cached and served to callers who sent another. `sort_query=False` works on 0.3.9 already.
 - Entries written by 0.3.x are not read by 0.4.0. They expire on their TTL; on Redis and memory you can remove them right after the upgrade with `await backend.clear_pattern("*|||*")`. That pattern matches any key containing `|||`, so check first that none of your own keys (a `CacheManager` key, say) does. Memcached cannot enumerate keys, so there they just expire.
diff --git a/fastapi_cachex/cache.py b/fastapi_cachex/cache.py
index b61608d..751a998 100644
--- a/fastapi_cachex/cache.py
+++ b/fastapi_cachex/cache.py
@@ -83,7 +83,9 @@ def build_cache_key(
         def per_user_key(request: Request) -> str:
             return build_cache_key(request, request.state.user_id)
 
-    ``|`` and ``%`` in the host, the path and every extra component are
+    The host is the ``Host`` header lower-cased, without an empty or default
+    port (``:80`` on http, ``:443`` on https), or ``unknown`` when there is
+    none. ``|`` and ``%`` in the host, the path and every extra component are
     percent-encoded (see ``escape_key_component``), so none of them can
     contain the separator and make one request's key equal another's. The
     query string is already URL-encoded and never contains ``|``.
diff --git a/fastapi_cachex/cache_key.py b/fastapi_cachex/cache_key.py
index c6f355b..d30bae5 100644
--- a/fastapi_cachex/cache_key.py
+++ b/fastapi_cachex/cache_key.py
@@ -20,6 +20,9 @@
 # Format tag, method, host, path and query: a key never has fewer components.
 _MIN_PARTS = 5
 
+# The port a scheme implies when the Host header names none.
+_DEFAULT_PORTS = {"http": "80", "https": "443", "ws": "80", "wss": "443"}
+
 # Characters that are live in a Redis glob pattern.
 _GLOB_SPECIAL = frozenset("*?[]\\")
 
@@ -44,6 +47,35 @@ def _query_component(request: Request, sort_query: bool) -> str:
     return urlencode(sorted(request.query_params.multi_items(), key=itemgetter(0)))
 
 
+def _host_component(request: Request) -> str:
+    """The ``Host`` header as it appears in the key.
+
+    Hostnames are case-insensitive (RFC 9110 section 4.2.3), and an empty port
+    or the scheme's default one (``:80`` for http, ``:443`` for https) names
+    the same origin as no port (RFC 3986 section 6.2.3). So the host is
+    lower-cased and such a port dropped, and every spelling of one origin
+    shares an entry. An IPv6 literal keeps its brackets. The scheme is the
+    one the app sees (the ASGI scope's ``scheme``, which ``request.url``
+    also uses); behind a TLS-terminating proxy that is ``http`` unless the
+    proxy's headers are applied. A missing ``Host`` header gives
+    ``unknown``.
+    """
+    host = request.headers.get("host")
+    if host is None:
+        return "unknown"
+    host = host.lower()
+    name, colon, port = host.rpartition(":")
+    # No colon, no name before it, or the last colon is inside an IPv6
+    # literal (``[::1]``) or an unbracketed one (not a valid Host, kept as
+    # sent): no port to drop.
+    bracketed = name.startswith("[") and name.endswith("]")
+    if not colon or not name or (":" in name and not bracketed):
+        return host
+    if port in ("", _DEFAULT_PORTS.get(request.scope.get("scheme", "http"))):
+        return name
+    return host
+
+
 def _component_text(component: str | int) -> str:
     """``component`` as key text: a ``str`` as is, an ``int`` in decimal.
 
@@ -117,7 +149,7 @@ def from_request(
         """
         return cls(
             method=request.method,
-            host=request.headers.get("host", "unknown"),
+            host=_host_component(request),
             path=request.url.path,
             query=_query_component(request, sort_query),
             extra=tuple(_component_text(component) for component in components),
diff --git a/i18n/zh-TW/docs/CACHE_FLOW.md b/i18n/zh-TW/docs/CACHE_FLOW.md
index 1e38d18..6e16505 100644
--- a/i18n/zh-TW/docs/CACHE_FLOW.md
+++ b/i18n/zh-TW/docs/CACHE_FLOW.md
@@ -68,7 +68,7 @@ cache_key = "|".join(
     [
         CacheKey.FORMAT_TAG,  # "http:v2"
         escape_key_component(request.method),
-        escape_key_component(request.headers.get("host", "unknown")),
+        escape_key_component(host),  # Host 標頭，已正規化（見下文）
         escape_key_component(request.url.path),
         query,
     ]
@@ -92,7 +92,7 @@ cache_key = "|".join(
 這個快取鍵格式讓每個維度各自獨立快取：
 
 - **方法隔離**：GET 與 POST 不共用快取（而且目前只有 GET 會進入快取流程）
-- **Host 隔離**：`example.com` 與 `api.example.com` 分開快取
+- **Host 隔離**：`example.com` 與 `api.example.com` 分開快取；`Example.com` 與（在 http 上的）`example.com:80` 都是 `example.com`
 - **路徑隔離**：每個端點有各自的項目
 - **查詢參數隔離**：同一端點上不同的查詢參數分開快取
 
diff --git a/i18n/zh-TW/docs/HTTP_CACHING.md b/i18n/zh-TW/docs/HTTP_CACHING.md
index 5d5d9e8..5c36b84 100644
--- a/i18n/zh-TW/docs/HTTP_CACHING.md
+++ b/i18n/zh-TW/docs/HTTP_CACHING.md
@@ -188,7 +188,9 @@ async def search(q: str, limit: int = 10):
 
 host 與路徑來自用戶端，因此其中的 `|` 與 `%` 會以百分比編碼寫入（`%7C` 與 `%25`）。含有 `|` 的 `Host` 標頭或路徑因此無法讓各段錯位，使某個請求的快取鍵與另一個請求相同。查詢字串本來就經過 URL 編碼。`clear_path()` 接受應用程式看到的路徑（`request.url.path`），並以同樣方式編碼；`clear_pattern()` 比對的是儲存的快取鍵，所以在模式中要把 `|` 寫成 `%7C`。0.3.8 之前兩者都照原樣儲存，因此升級後，host 或路徑含有 `|` 或 `%` 的項目會重新快取一次。
 
-host 仍是用戶端送來的任何值。除非應用程式前方的反向代理或負載平衡器已會拒絕未知的 host，否則請加上 Starlette 的 `TrustedHostMiddleware`，讓偽造的 `Host` 得到 `400`，而不是在快取中塞滿沒有其他人會請求的項目：
+host 會先經過正規化，讓同一個來源的各種寫法共用同一筆項目：轉為小寫（主機名稱不分大小寫），並去除空的連接埠或該 scheme 的預設連接埠（http 為 `:80`，https 為 `:443`）。在 http 上，`Example.com`、`example.com:80` 與 `example.com` 是同一個鍵 `example.com`；`example.com:8080` 保留連接埠，IPv6 位址則保留方括號（`[::1]:8000`）。scheme 以應用程式看到的為準：在終止 TLS 的代理之後，除非套用了代理的標頭（例如 `uvicorn --proxy-headers`），否則 scheme 是 `http`，因此這類代理送來的 `Host: example.com:443` 會保留連接埠。沒有 `Host` 標頭的請求使用 `unknown`。
+
+除此之外，host 仍是用戶端送來的任何值。除非應用程式前方的反向代理或負載平衡器已會拒絕未知的 host，否則請加上 Starlette 的 `TrustedHostMiddleware`，讓偽造的 `Host` 得到 `400`，而不是在快取中塞滿沒有其他人會請求的項目：
 
 ```python
 from starlette.middleware.trustedhost import TrustedHostMiddleware
diff --git a/i18n/zh-TW/docs/MIGRATING_0_4.md b/i18n/zh-TW/docs/MIGRATING_0_4.md
index 2ab1a01..f297094 100644
--- a/i18n/zh-TW/docs/MIGRATING_0_4.md
+++ b/i18n/zh-TW/docs/MIGRATING_0_4.md
@@ -251,7 +251,7 @@ CacheManagerProxy.set(CacheManager(lock=True))
 
 - 分隔符號改為單一的 `|`（`CACHE_KEY_SEPARATOR`）。
 - 鍵以格式標籤 `http:v2|`（`CacheKey.FORMAT_TAG`）開頭，讓下一次格式變更可以用模式移除舊鍵：`clear_pattern("http:v2|*")` 會移除這個格式的所有鍵。
-- 主機名稱會正規化：轉為小寫，並去除該 scheme 的預設連接埠（`:80`、`:443`）。
+- host 會正規化：轉為小寫，並去除空的連接埠或該 scheme 的預設連接埠（http 為 `:80`，https 為 `:443`）。
 - 過長的查詢字串（約超過 200 位元組）會以 `sha256:` 加上十六進位摘要儲存；路徑仍保持可讀。
 - 查詢參數會依名稱排序：`sort_query`（0.3.9 起可選用）在 `@cache`、`build_cache_key()` 與 `invalidate()` 中預設為 `True`，因此 `?b=2&a=1` 與 `?a=1&b=2` 共用同一筆項目。
 - 由單一、公開的 `CacheKey` 型別負責建立、編碼與解析鍵。`fastapi_cachex.routes` 中的 `CACHE_KEY_MIN_PARTS`、`CACHE_KEY_MAX_SPLIT` 與 `CACHE_KEY_MAX_PARTS` 已移除；請改用 `CacheKey.parse(key)` 讀取鍵的各段。
@@ -263,7 +263,7 @@ CacheManagerProxy.set(CacheManager(lock=True))
 
 需要修改的地方：
 
-- 直接寫出分隔符號的 `clear_pattern()` 模式（`"GET|||*|||/users/*"`）需要改寫。`clear_path()` 與 `invalidate()` 會自行組出鍵，不需要修改。
+- 直接寫出分隔符號的 `clear_pattern()` 模式（`"GET|||*|||/users/*"`）需要改寫，以大寫或帶預設連接埠寫出 host 的模式也一樣。`clear_path()` 與 `invalidate()` 會自行組出鍵，不需要修改。
 - 呼叫 `build_cache_key()` 的自訂 `key_builder` 會自動跟上。自行組出鍵的（以 `CACHE_KEY_SEPARATOR` 串接或直接寫死 `|||`）仍可以快取，也仍能搭配 `invalidate()` 與 `clear_pattern()`，但其鍵沒有 `http:v2` 標籤，因此 `clear_path()` 不再找得到它們，監控路由也不再列出它們。改用 `build_cache_key(request, *components)` 即可兩者都保留。
 - 回應取決於用戶端送出的查詢字串順序的處理函式（例如從 `request.url` 複製的自身連結或分頁連結、對原始查詢字串計算的簽章），請設定 `@cache(sort_query=False)`。否則第一位呼叫者的順序會被快取，並提供給送出其他順序的呼叫者。`sort_query=False` 在 0.3.9 就能使用。
 - 0.4.0 不會讀取 0.3.x 寫入的項目。這些項目會在 TTL 到期後過期；在 Redis 與記憶體後端上，可以在升級後立即以 `await backend.clear_pattern("*|||*")` 移除。這個模式會比對任何含有 `|||` 的鍵，因此請先確認你自己的鍵（例如 `CacheManager` 的鍵）都不含它。Memcached 無法列舉鍵，只能等它們過期。
diff --git a/tests/test_cache_key_type.py b/tests/test_cache_key_type.py
index 2798358..6f5382d 100644
--- a/tests/test_cache_key_type.py
+++ b/tests/test_cache_key_type.py
@@ -64,7 +64,7 @@ def test_build_cache_key_format_is_pinned() -> None:
     request = _request("/p|q", b"b=2&a=1", "Host|||x")
 
     assert build_cache_key(request, "a|b", 100, sort_query=True) == (
-        "http:v2|GET|Host%7C%7C%7Cx|/p%7Cq|a=1&b=2|a%7Cb|100"
+        "http:v2|GET|host%7C%7C%7Cx|/p%7Cq|a=1&b=2|a%7Cb|100"
     )
 
 
diff --git a/tests/test_host_normalization.py b/tests/test_host_normalization.py
new file mode 100644
index 0000000..8b2111c
--- /dev/null
+++ b/tests/test_host_normalization.py
@@ -0,0 +1,91 @@
+"""The host in an HTTP cache key is normalised (#265)."""
+
+import pytest
+from fastapi import FastAPI
+from fastapi import Request
+from fastapi.testclient import TestClient
+
+from fastapi_cachex import CacheKey
+from fastapi_cachex import cache
+from fastapi_cachex import invalidate
+from fastapi_cachex.backends import MemoryBackend
+from fastapi_cachex.proxy import BackendProxy
+from fastapi_cachex.types import CacheEntry
+
+
+def _request(host: str | None, scheme: str = "http") -> Request:
+    headers = [] if host is None else [(b"host", host.encode("latin-1"))]
+    return Request(
+        {
+            "type": "http",
+            "scheme": scheme,
+            "method": "GET",
+            "path": "/p",
+            "raw_path": b"/p",
+            "query_string": b"",
+            "headers": headers,
+        }
+    )
+
+
+@pytest.mark.parametrize(
+    ("host", "scheme", "expected"),
+    [
+        pytest.param("example.com", "http", "example.com", id="plain"),
+        pytest.param("Example.COM", "http", "example.com", id="case"),
+        pytest.param("example.com:80", "http", "example.com", id="http-default"),
+        pytest.param("example.com:443", "https", "example.com", id="https-default"),
+        pytest.param("example.com:443", "http", "example.com:443", id="http-443"),
+        pytest.param("example.com:80", "https", "example.com:80", id="https-80"),
+        pytest.param("example.com:8080", "http", "example.com:8080", id="other-port"),
+        pytest.param("example.com:", "http", "example.com", id="empty-port"),
+        pytest.param("Example.com:8080", "https", "example.com:8080", id="case-port"),
+        pytest.param("[::1]", "http", "[::1]", id="ipv6"),
+        pytest.param("[::1]:80", "http", "[::1]", id="ipv6-default"),
+        pytest.param("[::1]:8000", "http", "[::1]:8000", id="ipv6-port"),
+        pytest.param("[FE80::1]:443", "https", "[fe80::1]", id="ipv6-case"),
+        pytest.param("::1:80", "http", "::1:80", id="ipv6-unbracketed"),
+        pytest.param(":80", "http", ":80", id="port-only"),
+        pytest.param("example.com.", "http", "example.com.", id="trailing-dot"),
+        pytest.param("example.com:443", "wss", "example.com", id="wss-default"),
+        pytest.param("example.com:80", "ws", "example.com", id="ws-default"),
+        pytest.param("[::1]:", "http", "[::1]", id="ipv6-empty-port"),
+        pytest.param("host:80:80", "http", "host:80:80", id="two-ports"),
+        pytest.param("HOST:080", "http", "host:080", id="leading-zero-port"),
+        pytest.param("", "http", "", id="empty"),
+        pytest.param(None, "http", "unknown", id="missing"),
+    ],
+)
+def test_host_is_normalised(host: str | None, scheme: str, expected: str) -> None:
+    assert CacheKey.from_request(_request(host, scheme)).host == expected
+
+
+def test_spellings_of_one_origin_share_an_entry() -> None:
+    app = FastAPI()
+    backend = MemoryBackend()
+    BackendProxy.set(backend)
+    calls = 0
+
+    @app.get("/p")
+    @cache(ttl=60)
+    async def endpoint() -> dict[str, int]:
+        nonlocal calls
+        calls += 1
+        return {"calls": calls}
+
+    client = TestClient(app)
+    for host in ("example.com", "EXAMPLE.com", "example.com:80"):
+        assert client.get("/p", headers={"host": host}).json() == {"calls": 1}
+
+    assert list(backend.cache) == ["http:v2|GET|example.com|/p|"]
+
+
+async def test_invalidate_finds_the_entry_under_another_spelling() -> None:
+    backend = MemoryBackend()
+    BackendProxy.set(backend)
+    await backend.set(
+        CacheKey("GET", "example.com", "/p").to_str(), CacheEntry("e", b"v")
+    )
+
+    assert await invalidate(_request("Example.com:80")) is True
+    assert backend.cache == {}