BROCADE IP PRIMER

49F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 48BI4XGBI24CT X RX T X RX T X RX T X RXBI4XGBI24CT X RX T X RX T X RX T X RXBI24C6 12 18 2430 36 42 48BI24CBigIron RX-8NETWORKS1 7 13 1925 31 37 43BI24CBI24CBIBI24C4XGT X RX T X RX T X RX T X RXBIBI24C4XGT X RX T X RX T X RX T X RXAC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS AC OK DC OK ALMEJECT SYS49F Link 50FPower11F 2F 3F 4F32Console5476981110131215141716191821202322252427262928313033323534373639384140434245444746 48Chapter 2: TCP/IPNow, what if you have one router that is serving as a gateway for several broadcastdomains? You can still use a helper address, but how will the router keeptrack of which broadcast domain made the request? This is where you needDHCP Assist. This is configured on a Layer 2 switch. In a nutshell, you configurethe switch port that belongs to a particular broadcast domain to forwardDHCP requests stamped for their particular broadcast domain. Let's look at apicture.DHCP Assist192.168.100.0/24DHCP Assist10.0.0.0/8192.168.100.1/2410.0.0.1/24When the request is received by the Layer 2 switch (configured with DHCPAssist), it stamps the request so that the router knows which broadcastdomain made the request. Without this, the router may end up respondingwith addresses in the wrong broadcast domain! This doesn't come up veryoften, but when it does, you'll be glad to know it's there.Network Address Translation (NAT)Earlier in the chapter, we talked about private IP networks that are defined byRFC 1918. These addresses can be used by anyone, but they can never berouted across the Internet. This seemed like a good system. After all, you wantyour network to be private, right? It's funny how consumers change their mind.With Network Address Translation (NAT), you can use your private networklocally, and you can use the Internet. NAT would usually be configured on arouter that sits between your private network and the destination network(whether that is the Internet, a newly-merged business, etc.). NAT does nothave to be an RFC 1918 address being translated to a publicly-routable Internetaddress. It is simply one address being translated into another.The router is performing a swap. Let's say for example that you're using theRFC 1918 network 192.168.100.0/24. You have a client: 92.168.100.10. Youalso have a connection to the Internet, and a publicly-routed range of IPaddresses: 1.1.2.0/24. If your client needs to get out to the Internet, you couldconfigure your router to translate 192.168.100.10 to a public address, forexample, 1.1.2.10. Now, every time the client attempts to go out to the Internet,the router will translate the client's private address (192.168.100.10) tothe translated public address (1.1.2.10). Likewise, when replies or initial connectionscome in from the Internet to 1.1.2.10, the router will translate them to192.168.100.10. Notice that I chose to keep the last octet of the client's privateIP address and its public IP address the same (“.10”).62 Brocade IP Primer