chia a ch mng con theo VLSM, phng php ny s gip chng ta kim sot c s mng mi sinh ra, s mng dng, s mng d tha cn li, sau y ti s hng dn cc bn thc hin vic ny mt cch d dng bng v d minh ha. Trc ht, chng ta phi hiu r cu trc ca a ch IP v4 v ngha ca mt s khi nim: v d cc lp a ch IP v4, Net_id, host_id, Subnet Mask, gii a ch kh dng, a ch mng, chia thnh tho, chng ta cn nm r mt s khi nim v cng thc sau y:- Bit, byte.- Khi nim v s nh phn, thp phn.- Php ton AND- Cc bin i t nh phn sang thp phn, t thp phn sang nh phn.- Cu trc a ch IP, gii hn ca cc lp IP- Khi nim v default mask, mask, subnet, subneting ....!- Cc a ch ringLu :- a ch mng (subnet) : tt c cc bit dnh cho phn host bng 0- a ch broadcast: tt c cc bit dnh cho phn host bng 1.- a ch u tin hp l: l a ch lin sau a ch mng (subnet)- a ch cui cng hp l: l a ch lin trc a ch broadcast=> phi hiu r v phn bit khi nim n v m l g p dng cng thc cho ng- Cng thc:+ S subnet c to ra: 2m (m: s bit mn ca phn Host ID) (Ch : ng lcng thc ny phi l 2m 2 v phi loi tr i 2 mng u tin subnet zero vmng cui cng subnet broadcast, nhng vi cc dng Router hin nay ca Cisco h tr lnh Router(config)# ip subnet-zero do ta vn c th s dng 2 mng m khng phi loi tr b i)+ S host / subnet: 2n 2 (n: s bit cn li ca phn Host ID sau khi b mn m bit)+ Subnet Mask mi = Subnet Mask c + m (l s bit va b mn)+ a ch kh dng l cc a ch IP c th gn cho mi host, thit b(Lu :c nhiu cch hoc th thut tnh a ch mng con, nhng cch no cng phi da vo nn tng gc l s thay i cc bit mn sinh ra mng con mi, do tt nht chng ta nn tham kho theo phng php VLSM)V D MU 1:

Cho gii a ch 172.35.0.0/16 , hy Subnet cp cho cc mng con:A: 320 hostB: 115 hostC: 80 hostD: 30 hostE: 2 hostF: 2 hostG: 2 hosttheo phng php VLSM?Hng dn gii mu:- Theo u bi cho a ch ban u l X: 172.35.0.0/16=> i ra h nh phn ta c:10101100.00100011.00000000.0000000011111111.11111111.00000000.00000000(Phn gch chn chnh l phn bit host, vic chia t a ch trn thnh nhiu Subnet chnh l vic bin i hay gi l mn cc bit phn host_id chuyn thnh cc bit Net_id; Nhn vo s bit 1 ca a ch Subnet Mask ta s phn bit c danh gii: cc bit bn trn bit 1 chnh l Net_id, cc bit bn trn bit 0 l host_id)- B1:Theo VLSM th ta s phi chia X cho cc mng theo chiu gim dn, tc l chia cho mng c s host cao nht ri thp nht cui cng-> sp xp li ta c:+A: 320+B:115+C:80+D:30+E:2+F:2+G:2- B2:+Thc hin chia X cho mng A u tin, p dng cng thc: 2n- 2 320 => n=9 (chnh l s bit cn li cha b mn) => s bit mn l m= 32 (l tng s bit ca 1 a ch IP v4) 16 (s bit thuc phn Net_id ca a ch cho) 9 ( s bit cn li) = 7 => SM (Subnet Mask mi) = SM (Subnet Mask c) + m = 16 + 7 = 23 ( vit tt l /23) & s Subnet (mng con) c to ra l: 2m = 27 = 128 vi SM thay i t /16 thnh /23 (cc bit trong khong ny ca X chuyn sang Octetth 3) nn ta c