diff --git a/README.md b/README.md
index 7f6ecbf..4c57c2d 100644
--- a/README.md
+++ b/README.md
@@ -78,14 +78,12 @@ server_types:
     boot_disk: "/dev/nvme0n1"
     management_nic: "enp2s0"
     gpu: false
-    hostname_prefix: "pve-ms"
 
   minisforum-um790:
     display_name: "Minisforum UM790 Pro"
     boot_disk: "/dev/nvme0n1"
     management_nic: "enp1s0"
     gpu: true
-    hostname_prefix: "pve-um"
 ```
 
 ### Run
diff --git a/deploy/server-types.example.yaml b/deploy/server-types.example.yaml
index 5519502..79e6419 100644
--- a/deploy/server-types.example.yaml
+++ b/deploy/server-types.example.yaml
@@ -4,7 +4,6 @@ server_types:
     boot_disk: "/dev/nvme0n1"
     management_nic: "enp2s0"
     gpu: false
-    hostname_prefix: "pve-ms"
     expected_nics:
       - name: "enp2s0"
         speed: "2500"
@@ -16,7 +15,6 @@ server_types:
     boot_disk: "/dev/nvme0n1"
     management_nic: "enp1s0"
     gpu: true
-    hostname_prefix: "pve-um"
     expected_nics:
       - name: "enp1s0"
         speed: "2500"
diff --git a/internal/api/hosts.go b/internal/api/hosts.go
index 726dcb9..298265c 100644
--- a/internal/api/hosts.go
+++ b/internal/api/hosts.go
@@ -65,6 +65,14 @@ func (a *HostAPI) Create(w http.ResponseWriter, r *http.Request) {
 		writeJSONErr(w, http.StatusBadRequest, "unknown server_type")
 		return
 	}
+	if existing, err := a.Hosts.GetByHostname(r.Context(), req.Hostname); err == nil && existing != nil {
+		writeJSONErr(w, http.StatusConflict, "a host with this hostname already exists")
+		return
+	}
+	if existing, err := a.Hosts.GetByMAC(r.Context(), req.MAC); err == nil && existing != nil {
+		writeJSONErr(w, http.StatusConflict, "a host with this MAC address already exists")
+		return
+	}
 
 	id, err := a.Hosts.Create(r.Context(), model.Host{
 		Hostname:   req.Hostname,
diff --git a/internal/api/smoke_test.go b/internal/api/smoke_test.go
index 7e88da2..3c8e14a 100644
--- a/internal/api/smoke_test.go
+++ b/internal/api/smoke_test.go
@@ -134,7 +134,6 @@ func mustLoadServerTypes(t *testing.T, dir string) *config.ServerTypeRegistry {
     boot_disk: "/dev/sda"
     management_nic: "eth0"
     gpu: false
-    hostname_prefix: "pve-test"
 `)
 	if err := writeTestFile(path, content); err != nil {
 		t.Fatal(err)
@@ -337,6 +336,82 @@ func TestOperationNotFound(t *testing.T) {
 	resp.Body.Close()
 }
 
+func TestDuplicateHostnameRejected(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.Close()
+
+	body := `{"hostname":"pve-dup-01","mac":"aa:bb:cc:dd:ee:d1","server_type":"test-type"}`
+	resp, err := http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("first create: got %d, want %d", resp.StatusCode, http.StatusCreated)
+	}
+	resp.Body.Close()
+
+	body = `{"hostname":"pve-dup-01","mac":"aa:bb:cc:dd:ee:d2","server_type":"test-type"}`
+	resp, err = http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("dup hostname: got %d, want %d", resp.StatusCode, http.StatusConflict)
+	}
+	var errResp map[string]any
+	json.NewDecoder(resp.Body).Decode(&errResp)
+	resp.Body.Close()
+	if msg, _ := errResp["error"].(string); msg != "a host with this hostname already exists" {
+		t.Fatalf("dup hostname error = %q, want specific hostname message", msg)
+	}
+}
+
+func TestDuplicateMACRejected(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.Close()
+
+	body := `{"hostname":"pve-mac-01","mac":"aa:bb:cc:dd:ee:e1","server_type":"test-type"}`
+	resp, err := http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusCreated {
+		t.Fatalf("first create: got %d, want %d", resp.StatusCode, http.StatusCreated)
+	}
+	resp.Body.Close()
+
+	body = `{"hostname":"pve-mac-02","mac":"aa:bb:cc:dd:ee:e1","server_type":"test-type"}`
+	resp, err = http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("dup MAC: got %d, want %d", resp.StatusCode, http.StatusConflict)
+	}
+	var errResp map[string]any
+	json.NewDecoder(resp.Body).Decode(&errResp)
+	resp.Body.Close()
+	if msg, _ := errResp["error"].(string); msg != "a host with this MAC address already exists" {
+		t.Fatalf("dup MAC error = %q, want specific MAC message", msg)
+	}
+}
+
+func TestDuplicateHostnameCaseInsensitive(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.Close()
+
+	body := `{"hostname":"pve-case-01","mac":"aa:bb:cc:dd:ee:c1","server_type":"test-type"}`
+	resp, _ := http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	resp.Body.Close()
+
+	body = `{"hostname":"PVE-CASE-01","mac":"aa:bb:cc:dd:ee:c2","server_type":"test-type"}`
+	resp, _ = http.Post(ts.URL+"/api/hosts", "application/json", bytes.NewBufferString(body))
+	if resp.StatusCode != http.StatusConflict {
+		t.Fatalf("case-insensitive dup: got %d, want %d", resp.StatusCode, http.StatusConflict)
+	}
+	resp.Body.Close()
+}
+
 func itoa(i int64) string {
 	return fmt.Sprintf("%d", i)
 }
diff --git a/internal/api/ui.go b/internal/api/ui.go
index 18c5c28..1daa01f 100644
--- a/internal/api/ui.go
+++ b/internal/api/ui.go
@@ -74,6 +74,22 @@ func (u *UI) CreateHost(w http.ResponseWriter, r *http.Request) {
 		}))
 		return
 	}
+	if existing, err := u.Hosts.GetByHostname(r.Context(), hostname); err == nil && existing != nil {
+		errs = append(errs, "A host with this hostname already exists")
+	}
+	if existing, err := u.Hosts.GetByMAC(r.Context(), mac); err == nil && existing != nil {
+		errs = append(errs, "A host with this MAC address already exists")
+	}
+	if len(errs) > 0 {
+		types := u.ServerTypes.Keys()
+		renderHTML(w, hostFormPage(types, strings.Join(errs, "; "), &model.Host{
+			Hostname:   hostname,
+			MAC:        mac,
+			ServerType: serverType,
+			Notes:      notes,
+		}))
+		return
+	}
 
 	_, err := u.Hosts.Create(r.Context(), model.Host{
 		Hostname:   hostname,
diff --git a/internal/model/model.go b/internal/model/model.go
index 2a85175..332d5dd 100644
--- a/internal/model/model.go
+++ b/internal/model/model.go
@@ -85,13 +85,12 @@ type ActivityEntry struct {
 }
 
 type ServerType struct {
-	Key            string
-	DisplayName    string   `yaml:"display_name"`
-	BootDisk       string   `yaml:"boot_disk"`
-	ManagementNIC  string   `yaml:"management_nic"`
-	GPU            bool     `yaml:"gpu"`
-	HostnamePrefix string   `yaml:"hostname_prefix"`
-	ExpectedNICs   []NICDef `yaml:"expected_nics"`
+	Key           string
+	DisplayName   string   `yaml:"display_name"`
+	BootDisk      string   `yaml:"boot_disk"`
+	ManagementNIC string   `yaml:"management_nic"`
+	GPU           bool     `yaml:"gpu"`
+	ExpectedNICs  []NICDef `yaml:"expected_nics"`
 }
 
 type NICDef struct {
diff --git a/internal/store/hosts.go b/internal/store/hosts.go
index f89a55c..bb0b6fc 100644
--- a/internal/store/hosts.go
+++ b/internal/store/hosts.go
@@ -87,6 +87,18 @@ func (s *Hosts) GetByMAC(ctx context.Context, mac string) (*model.Host, error) {
 	return &h, nil
 }
 
+func (s *Hosts) GetByHostname(ctx context.Context, hostname string) (*model.Host, error) {
+	row := s.DB.QueryRowContext(ctx, `SELECT `+hostColumns+` FROM hosts WHERE LOWER(hostname) = ?`, normalizeHostname(hostname))
+	var h model.Host
+	if err := scanHost(row, &h); err != nil {
+		if err == sql.ErrNoRows {
+			return nil, ErrNotFound
+		}
+		return nil, fmt.Errorf("get host by hostname: %w", err)
+	}
+	return &h, nil
+}
+
 func (s *Hosts) UpdateState(ctx context.Context, id int64, state model.HostState) error {
 	res, err := s.DB.ExecContext(ctx, `UPDATE hosts SET state = ?, updated_at = strftime('%Y-%m-%dT%H:%M:%SZ','now') WHERE id = ?`, state, id)
 	if err != nil {
@@ -143,3 +155,7 @@ func (s *Hosts) Delete(ctx context.Context, id int64) error {
 func normalizeMAC(m string) string {
 	return strings.ToLower(strings.TrimSpace(m))
 }
+
+func normalizeHostname(h string) string {
+	return strings.ToLower(strings.TrimSpace(h))
+}