Seems to follow the common DLink layout: UBoot at 0-30000, UBoot-env 30000-40000, Factory 40000-50000, Firmware 50000-end.
$ binwalk flash.bin
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 uImage header, header size: 64 bytes, header CRC: 0x14C106AB, created: 2011-08-30 03:06:28, image size: 137172 bytes, Data Address: 0x80200000, Entry Point: 0x80200000, data CRC: 0x6D684DFC, OS: Linux, CPU: MIPS, image type: Standalone Program, compression type: none, image name: "SPI Flash Image"
107184 0x1A2B0 U-Boot version string, "U-Boot 1.1.3 (Aug 30 2011 - 11:06:24) (ALPHA)"
107808 0x1A520 CRC32 polynomial table, little endian
123984 0x1E450 HTML document header
124339 0x1E5B3 HTML document footer
124348 0x1E5BC HTML document header
124540 0x1E67C HTML document footer
124708 0x1E724 HTML document header
125401 0x1E9D9 HTML document footer
262160 0x40010 gzip compressed data, maximum compression, from Unix, last modified: 1999-12-31 16:17:22
327680 0x50000 SEAMA firmware header, big endian, meta size: 36, image size: 4603936
327712 0x50020 Unix path: /dev/mtdblock/2
327744 0x50040 LZMA compressed data, properties: 0x5D, dictionary size: 33554432 bytes, uncompressed size: 3805904 bytes
1638464 0x190040 PackImg section delimiter tag, little endian size: 4207104 bytes; big endian size: 3293184 bytes
1638496 0x190060 Squashfs filesystem, little endian, version 4.0, compression:lzma, size: 3289945
Flash Macronix MX25L6406E (https://www.macronix.com/Lists/Datasheet/Attachments/8630/MX25L6406E,%203V,%2064Mb,%20v1.9.pdf)
nearly identical to MX25L6405D (https://www.mxic.com.tw/Lists/Datasheet/Attachments/8570/MX25L6405D,%203V,%2064Mb,%20v1.5.pdf)
used by DIR 620 already supported by OpenWrt.