數(shù)據(jù)報(bào)轉(zhuǎn)發(fā)計(jì)算下跳地址專題培訓(xùn)課件

數(shù)據(jù)報(bào)轉(zhuǎn)發(fā)計(jì)算下跳地址在劃分子網(wǎng)的情況下路由器轉(zhuǎn)發(fā)分組的算法(1)從收到的分組的首部提取目的IP地址D。(2)先用各網(wǎng)絡(luò)的子網(wǎng)掩碼和D逐位相“與”，看是否和相應(yīng)的網(wǎng)絡(luò)地址匹配。若匹配，則將分組直接交付。否則就是間接交付，執(zhí)行(3)。(3)若路由表中有目的地址為D的特定主機(jī)路由，則將分組傳送給指明的下一跳路由器；否則，執(zhí)行(4)。(4)對(duì)路由表中的每一行的子網(wǎng)掩碼和D逐位相“與”，若其結(jié)果與該行的目的網(wǎng)絡(luò)地址匹配，則將分組傳送給該行指明的下一跳路由器；否則，執(zhí)行(5)。(5)若路由表中有一個(gè)默認(rèn)路由，則將分組傳送給路由表中所指明的默認(rèn)路由器；否則，執(zhí)行(6)。(6)報(bào)告轉(zhuǎn)發(fā)分組出錯(cuò)。128.30.33.10128.30.33.13H1子網(wǎng)1：

網(wǎng)絡(luò)地址128.30.33.0子網(wǎng)掩碼255.255.255.128128.30.33.130R1的路由表（未給出默認(rèn)路由器）R11R2子網(wǎng)2：網(wǎng)絡(luò)地址128.30.33.128子網(wǎng)掩碼255.255.255.128H2128.30.33.13801128.30.33.129H3128.30.36.2子網(wǎng)3：網(wǎng)絡(luò)地址128.30.36.0子網(wǎng)掩碼255.255.255.0128.30.36.12【例4-4】已知互聯(lián)網(wǎng)和路由器R1中的路由表。主機(jī)H1向H2發(fā)送分組。試討論R1收到H1

向H2發(fā)送的分組后查找路由表的過(guò)程。主機(jī)H1要發(fā)送分組給H2

128.30.33.10R1的路由表（未給出默認(rèn)路由器）128.30.33.13H1子網(wǎng)1：網(wǎng)絡(luò)地址128.30.33.0子網(wǎng)掩碼255.255.255.128128.30.33.130R11R2子網(wǎng)2：網(wǎng)絡(luò)地址128.30.33.128子網(wǎng)掩碼255.255.255.128H2128.30.33.13801128.30.33.129H3128.30.36.2子網(wǎng)3：網(wǎng)絡(luò)地址128.30.36.0子網(wǎng)掩碼255.255.255.0128.30.36.12要發(fā)送的分組的目的IP地址：128.30.33.138128.30.33.10H1子網(wǎng)1：網(wǎng)絡(luò)地址128.30.33.0子網(wǎng)掩碼255.255.255.128128.30.33.130R11R2子網(wǎng)2：網(wǎng)絡(luò)地址128.30.33.128子網(wǎng)掩碼255.255.255.128H2128.30.33.13128.30.33.13801128.30.33.129H3128.30.36.2子網(wǎng)3：網(wǎng)絡(luò)地址128.30.36.0子網(wǎng)掩碼255.255.255.0128.30.36.12主機(jī)H1首先將

本子網(wǎng)的子網(wǎng)掩碼255.255.255.128

與分組的

IP

地址128.30.33.138逐比特相“與”(AND操作)255.255.255.128AND128.30.33.138的計(jì)算255就是二進(jìn)制的全1，因此255ANDxyz=xyz，這里只需計(jì)算最后的128AND138即可。128→10000000138→10001010逐比特AND操作后：10000000→128255.255.255.128128.30.33.138128.30.33.128逐比特AND操作H1

的網(wǎng)絡(luò)地址因此H1必須把分組傳送到路由器R1

然后逐項(xiàng)查找路由表128.30.33.10R1的路由表（未給出默認(rèn)路由器）128.30.33.13H1子網(wǎng)1：網(wǎng)絡(luò)地址128.30.33.0子網(wǎng)掩碼255.255.255.128128.30.33.130R11R2子網(wǎng)2：網(wǎng)絡(luò)地址128.30.33.128子網(wǎng)掩碼255.255.255.128H2128.30.33.13801128.30.33.129H3128.30.36.2子網(wǎng)3：網(wǎng)絡(luò)地址128.30.36.0子網(wǎng)掩碼255.255.255.0128.30.36.12路由器R1收到分組后就用路由表中第1個(gè)項(xiàng)目的

子網(wǎng)掩碼和128.30.33.138逐比特AND操作128.30.33.10R1的路由表（未給出默認(rèn)路由器）128.30.33.13H1子網(wǎng)1：網(wǎng)絡(luò)地址128.30.33.0子網(wǎng)掩碼255.255.255.128128.30.33.130R11R2子網(wǎng)2：網(wǎng)絡(luò)地址128.30.33.128子網(wǎng)掩碼255.255.255.128H2128.30.33.13801128.30.33.129H3128.30.36.2子網(wǎng)3：網(wǎng)絡(luò)地址128.30.36.0子網(wǎng)掩碼255.255.255.0128.30.36.12255.255.255.128AND128.30.33.138=128.30.33.128不匹配!（因?yàn)?28.30.33.128與路由表中的128.30.33.0不一致）R1收到的分組的目的IP地址：128.30.33.138不一致路由器R1再用路由表中第2個(gè)項(xiàng)目的

子網(wǎng)掩碼和128.30.33.138逐比特AND操作128.30.33.10R1的路由表（未給出默認(rèn)路由器）128.30.33.13H1子網(wǎng)1：網(wǎng)絡(luò)地址128.30.33.0子網(wǎng)掩碼255.255.255.128128.30.33.130R11R2子網(wǎng)2：網(wǎng)絡(luò)地址128.30.33.128子網(wǎng)掩碼255.255.255.128H2128.30.33.13801128.30.33.129H3128.30.36.2子網(wǎng)3：網(wǎng)絡(luò)地址128.30.36.0子網(wǎng)掩碼255.255.255.0128.30.36.12255.255.255.128AND128.30.33.138=128.30.33.128匹配!這表明子網(wǎng)2就是收到的分組所要尋找的目的網(wǎng)絡(luò)R1收到的分組的目的IP地址：128.30.33.138一致!設(shè)某路由器建立了如下路由表（這三列分別是目的網(wǎng)絡(luò)、子網(wǎng)掩碼和下一跳路由器，若直接交付則最后一列表示應(yīng)當(dāng)從哪一個(gè)接口轉(zhuǎn)發(fā)出去）：128.96.39.0255.255.255.128接口0128.96.39.128255.255.255.128接口1128.96.40.0255.255.255.128R2192.4.153.0255.255.255.192R3*（默認(rèn)）R4現(xiàn)共收到5個(gè)分組，其目的站IP地址分別為：（1）128.96.39.10（2）128.96.40.12（3）128.96.40.151（4）192.4.153.17（5）192.4.153.90試分別計(jì)算其下一跳。解：（1）分組的目的站IP地址為：128.96.39.10。先與子網(wǎng)掩碼255.255.255.128相與，得128.96.39.0，可見(jiàn)該分組經(jīng)接口0轉(zhuǎn)發(fā)。（2）分組的目的IP地址為：128.96.40.12。與子網(wǎng)掩碼255.255.255.128相與得128.96.40.0，不等于128.96.39.0。與子網(wǎng)掩碼255.255.255.128相與得128.96.40.0，經(jīng)查路由表可知，該項(xiàng)分組經(jīng)R2轉(zhuǎn)發(fā)。（3）分組的目的IP地址為：128.96.40.151，與子網(wǎng)掩碼255.255.255.128相與后得128.96.40.128，與子網(wǎng)掩碼255.255.255.192相與后得128.96.40.128，經(jīng)查路由表知，該分組轉(zhuǎn)發(fā)選擇默認(rèn)路由，經(jīng)R4轉(zhuǎn)發(fā)。（4）分組的目的IP地址為：192.4.153.17。與子網(wǎng)掩碼255.255.25