diff --git a/config/cluster.nix b/config/cluster.nix
index b6aa9d0..bb43be4 100644
--- a/config/cluster.nix
+++ b/config/cluster.nix
@@ -114,7 +114,6 @@
         server27 = "2a00:8180:2c00:284::127";
         server28 = "2a00:8180:2c00:284::128";
         server29 = "2a00:8180:2c00:284::129";
-        server30 = "2a00:8180:2c00:284::130";
       };
     };
     subnet4 = "172.20.77.0/27";
diff --git a/config/priv.nix b/config/priv.nix
index 3557748..19ead56 100644
--- a/config/priv.nix
+++ b/config/priv.nix
@@ -730,7 +730,7 @@
       hosts4 = { priv39-gw = "172.20.77.129"; };
       hosts6 = {
         dn42 = { priv39-gw = "fd23:42:c3d2:5e6::1"; };
-        up4 = { priv39-gw = "2a00:8180:2c00:2e5::1"; };
+        up4 = { priv39-gw = "2a00:8180:2c00:2e6::1"; };
       };
       subnet4 = "172.20.77.128/28";
       subnets6 = {
@@ -774,7 +774,7 @@
       hosts4 = { priv40-gw = "172.20.77.65"; };
       hosts6 = {
         dn42 = { priv40-gw = "fd23:42:c3d2:5e7::1"; };
-        up4 = { priv40-gw = "2a00:8180:2c00:2e6::1"; };
+        up4 = { priv40-gw = "2a00:8180:2c00:2e7::1"; };
       };
       subnet4 = "172.20.77.64/28";
       subnets6 = {
@@ -796,7 +796,7 @@
       hosts4 = { priv41-gw = "172.20.77.193"; };
       hosts6 = {
         dn42 = { priv41-gw = "fd23:42:c3d2:5e8::1"; };
-        up4 = { priv41-gw = "2a00:8180:2c00:2e7::1"; };
+        up4 = { priv41-gw = "2a00:8180:2c00:2e8::1"; };
       };
       subnet4 = "172.20.77.192/28";
       subnets6 = {
@@ -818,7 +818,7 @@
       hosts4 = { priv42-gw = "172.20.76.33"; };
       hosts6 = {
         dn42 = { priv42-gw = "fd23:42:c3d2:5e9::1"; };
-        up4 = { priv42-gw = "2a00:8180:2c00:2e8::1"; };
+        up4 = { priv42-gw = "2a00:8180:2c00:2e9::1"; };
       };
       subnet4 = "172.20.76.32/28";
       subnets6 = {
diff --git a/config/serv.nix b/config/serv.nix
index 0319d06..c5ca366 100644
--- a/config/serv.nix
+++ b/config/serv.nix
@@ -9,7 +9,6 @@
       stats = "172.20.73.3";
       radius = "172.20.73.4";
       zeit = "172.20.73.5";
-      ntp = "172.20.73.5";
       minecraft = "172.20.73.6";
       used1 = "172.20.73.7";
       dnscache = "172.20.73.8";
@@ -37,7 +36,6 @@
       used16 = "172.20.73.30";
       used17 = "172.20.73.31";
       scrape = "172.20.73.32";
-      used18 = "172.20.73.32";
       used19 = "172.20.73.33";
       used20 = "172.20.73.34";
       used21 = "172.20.73.35";
diff --git a/nix/lib/config/options.nix b/nix/lib/config/options.nix
index 7503c49..8387ec3 100644
--- a/nix/lib/config/options.nix
+++ b/nix/lib/config/options.nix
@@ -556,8 +556,49 @@ in
       (reportCollisions "IPv6 subnet" (x: builtins.attrValues x.subnets6) config.site.net) ++
       ospfUpstreamXorGw;
 
-  config.assertions = map (name: {
-    assertion = ! config.site.net ? ${name};
-    message = "Host \"${name}\" must be named differently if net \"${name}\" exists.";
-  }) (builtins.attrNames config.site.hosts);
+  config.assertions =
+    # Duplicate host/net name check
+    map (name: {
+      assertion = ! config.site.net ? ${name};
+      message = "Host \"${name}\" must be named differently if net \"${name}\" exists.";
+    }) (builtins.attrNames config.site.hosts)
+    ++
+    # Duplicate address check
+    (let
+      addrHosts =
+        builtins.foldl' (result: { hosts4, ... }:
+          builtins.foldl' (result: host:
+            let
+              addr = hosts4.${host};
+            in
+              if result ? ${addr}
+              then result // {
+                "${addr}" = result.${addr} ++ [ host ];
+              }
+              else result // {
+                "${addr}" = [ host ];
+              }
+          ) result (builtins.attrNames hosts4)
+        ) {} (builtins.attrValues config.site.net)
+        //
+        builtins.foldl' (result: net:
+          builtins.foldl' (result: ctx:
+            builtins.foldl' (result: host:
+              let
+                addr = config.site.net.${net}.hosts6.${ctx}.${host};
+              in
+                if result ? ${addr}
+                then result // {
+                  "${addr}" = result.${addr} ++ [ host ];
+                }
+                else result // {
+                  "${addr}" = [ host ];
+                }
+            ) result (builtins.attrNames config.site.net.${net}.hosts6.${ctx})
+          ) result (builtins.attrNames config.site.net.${net}.hosts6)
+        ) {} (builtins.attrNames config.site.net);
+    in map (addr: {
+      assertion = builtins.length addrHosts.${addr} == 1;
+      message = "Address ${addr} is assigned to more than one host: ${lib.concatStringsSep " " addrHosts.${addr}}";
+    }) (builtins.attrNames addrHosts));
 }