User's Manual

Chapter 10 Network Address Translation (NAT)
B222s User’s Guide
101
10.5.2 What NAT Does
I n t he sim plest form , NAT changes the sour ce I P address in a packet received from a subscriber
( t he inside local address) t o anot her ( the inside global address) before forwarding t he packet t o t he
WAN side. When t he response com es back, NAT t ranslat es the destinat ion address ( the inside
global addr ess) back t o t he inside local address before forwarding it t o t he original inside host . Not e
that t he I P address ( eit her local or global) of an out side host is never changed.
The global I P addresses for t he inside hosts can be eit her st atic or dynam ically assigned by t he I SP.
I n addit ion, you can designat e servers, for exam ple, a web ser ver and a Telnet server, on your local
network and m ake t hem accessible t o the outside world. I f you do not define any servers, NAT
offers t he additional benefit of firewall prot ection. With no servers defined, your LTE Device filt ers
out all incom ing inquiries, t hus prevent ing int ruders from probing your net work. For m ore
inform ation on I P address t ranslat ion, refer to RFC 1631, The I P Network Address Translator ( NAT) .
10.5.3 How NAT Works
Each packet has t wo addresses – a sour ce address and a destinat ion address. For outgoing packet s,
the I LA ( I nside Local Address) is the source address on t he LAN, and t he I GA ( I nside Global
Address) is t he source address on t he WAN. For incom ing packets, t he I LA is the dest inat ion
address on t he LAN, and t he I GA is t he dest ination address on t he WAN. NAT m aps privat e ( local)
I P addr esses t o globally unique ones r equired for com m unication wit h hosts on ot her networks. I t
replaces t he original I P source address ( and TCP or UDP source port num bers for Many- to- One and
Many-t o-Many Overload NAT m apping) in each packet and t hen forwards it to t he I nternet . The LTE
Device keeps t rack of the original addresses and port num bers so incom ing reply packet s can have
their original values restored. The following figure illust rat es this.
Figure 57 How NAT Works
192.168.1.13
192.168.1.10
192.168.1.11
192.168.1.12
SA
192.168.1.10
SA
IGA1
Inside Local
IP Address
192.168.1.10
192.168.1.11
192.168.1.12
192.168.1.13
Inside Global
IP Address
IGA 1
IGA 2
IGA 3
IGA 4
NAT Table
WAN
LAN
Inside Local
Address (ILA)
Inside Global
Address (IGA)