in realtà la cosa è un po più complicata..
ho realizzato un software che registra e il traffico generato da una rete suddiviso per subnet.... quindi per ogni subnet ho tutti gli ip ad esso assoiciati.

la tabella è cosi composta

sul db ho il traffico generato

id_movimento---- ip ----------------------mac-------------------id_rete
1 ----------------192.168.0.1------------ 00:1c:b0:78:fc:0c-----192.168.0.0
2 ----------------192.168.0.1 ------------00:1c:b0:78:fc:0c-----192.168.0.0
4 ----------------192.168.0.2------------ 00:30:6e:c3:45:62-----192.168.0.0
5 ----------------192.168.0.1------------ 00:1c:b0:78:fc:0c-----192.168.0.0
6 ----------------192.168.0.2------------ 00:30:6e:c3:45:62-----192.168.0.0
7 ----------------192.168.0.1------------ 00:1c:b0:78:fc:0c------192.168.0.0
8 ----------------192.168.0.2------------ 00:16:17:a6:68:0b------192.168.0.0
7 ----------------192.168.0.1------------ 00:14:a5:70:95:96 -----192.168.0.0

8 ----------------192.168.1.1------------ 00:1c:b1:78:fc:0c-----192.168.1.0
9 ----------------192.168.1.1 ------------00:1c:b1:78:fc:0c-----192.168.1.0
10 ----------------192.168.1.2------------ 00:20:7e:c3:45:62-----192.168.1.0
11 ----------------192.168.1.1------------ 00:1c:b1:78:fc:0c-----192.168.1.0
12 ----------------192.168.1.2------------ 00:20:7e:c3:45:62-----192.168.1.0
13 ----------------192.168.1.1------------ 00:1c:b1:78:fc:0c------192.168.1.0
14 ----------------192.168.1.2------------ 00:16:19:b6:68:0b------192.168.1.0
15 ----------------192.168.1.1------------ 00:20:a5:71:95:96 -----192.168.1.0

io dovrei fare una select del tipo

SELECT ip, id_rete, max(id_movimento), mac FROM movimenti group by id_rete, ip;

quindi deve prima raggrupparmi per id di rete dopo per ip, ma in questo modo ottengo

1 ----------------192.168.0.1------------ 00:1c:b0:78:fc:0c-----192.168.0.0
4 ----------------192.168.0.2------------ 00:30:6e:c3:45:62-----192.168.0.0
8 ----------------192.168.1.1------------ 00:1c:b1:78:fc:0c-----192.168.1.0
10 ----------------192.168.1.2------------ 00:20:7e:c3:45:62-----192.168.1.0

invece vorrei ottenere

7 ----------------192.168.0.1------------ 00:14:a5:70:95:96 -----192.168.0.0
8 ----------------192.168.0.2------------ 00:16:17:a6:68:0b------192.168.0.0
15 ----------------192.168.1.1------------ 00:20:a5:71:95:96 -----192.168.1.0
14 ----------------192.168.1.2------------ 00:16:19:b6:68:0b------192.168.1.0

se invece faccio

SELECT ip, id_rete, max(id_movimento), mac FROM movimenti group by id_rete, ip order by id_movimento DESC;

ottengo questo

10 ----------------192.168.1.2------------ 00:20:7e:c3:45:62-----192.168.1.0
8 ----------------192.168.1.1------------ 00:1c:b1:78:fc:0c-----192.168.1.0
4 ----------------192.168.0.2------------ 00:30:6e:c3:45:62-----192.168.0.0
1 ----------------192.168.0.1------------ 00:1c:b0:78:fc:0c-----192.168.0.0